Truth be told there are both pros and cons:

How will your swing be on the day? Will you swing at your natural pace or will it quicken as you try to impress the fitter? The fitter himself is he expertly trained or is he someone who works in a store that has an agenda on which club will make them the most profit?

However if you manage to replicate your normal swing and are fitted by the correct person then it is an invaluable experience. You will probably pick up distance and almost certainly achieve greater dispersion levels.

Whats the harm in giving it a try

You pointed out the first reason to get fit - you tweak your swing. A second reason to get fit, that with any of the multiple no-upcharge shafts, a custom set from Titleist will have the same per club price as off the rack. Titleist is not a mass merchandiser. Flex descriptions are not scientific. A good fitter will maximize your shot attack for launch, landing, and dispersion. Even if 2 shafts are designated stiff, with different kick points, the shots will differ. Once the shaft, length, loft, and lie have been determined, the order goes to Titleist where each set is individually prepared to those specs. You can also mix/match between the various heads (AP1, AP2, etc.) along with Vokey wedges, hybrids and fairways to match your needs for your bag. Order as many or few clubs as you want. Specified correctly, your fitter shouldn't need to adjust your set and that could jeopardize your warranty if they did. It's not like buying a 3-PW and having the loft bent in the store. Your set will be delivered the same as if you were Adam Scott.

I'll add that a Titleist fitting is just a fun experience. There is no commitment to buy anything. The fitter will give you a brief overview of what he/she will do and keep you informed as the fitting goes along. Go there with your everyday swing, and any time you hit a shot you're not proud of, tell him and he will discard the shot. Its an objective look at what you have and where some improvements could be found.

Here's a brief idea of how the process works based on my Titleist Thursday experience for driver and wedges. The fitter analyzed my bag, asked about my game and where I wanted to improve. He asked about my goals for the fitting, in my case I thought that I needed a higher launch with the driver, and the wedges I had I bought one at a time and needed reviewed. We reviewed briefly my working with a pro. Then I hit some balls with current clubs using TrackMan to show the fitter some numbers to work with.

For the driver, the fitter started a club/shaft tryout, hitting more balls under TrackMan. The fitter then gave me a best choice for best results based on a numbers comparison between current clubs and a new driver. I could see shot dispersion, spin rates, distances, and other comparisons between current and proposed changes. I could see how a new 915 would improve my game from the tee.

For the wedges the fitter noted that my SM5's were worn, something I hadn't noticed. The drill for wedges was a little different. The fitter asked me after a few shots if I was a picker and I confessed that I am because hitting down is hard on my wrists because through the summer my everyday course has hard clay under the grass. So he worked with me and my picking with the wedges. The SM6 numbers were a significant improvement.

My fitter made club change recommendations and the reasoning for the changes.
He gave me an accurate estimate of the cost. I liked what I saw and made the upgrade.

So after two weeks with a new 915 driver and SM6 wedges I'm real happy with the results so far. Drives are straighter and higher, getting me to my target much better. I'm still trying to fine tune my yardages with the wedges, things are going very well, the 50 and 60 degree wedges are competing for favorite club in the bag.

A fun experience at the fitting and even more fun on course. Hope this helped some. I should also note that I am a senior and struggling with the aging process.

The experience sounds great; I do have a question. Will Titleist keep track of your fitting experience and offer upgrades to your clubs as you advance in your game; or replace damaged or worn equipment like some other fitters do? What does the after fitting experience consist of?

Using a Titleist Thursday or a TPI/Manchester Lane fitting (Titleist employees/contractors) the specifications will be filed to your TT profile. Going to a certified fitter, you should request hard copies of the readouts (assuming Flight Scope or Trackman assistance) and the specifications for your clubs. Be sure to include grip, any loft/lie adjustments, and swing weight preferences while ordering. Although Titleist can track your specifications by the engraved number, if the order is not transmitted accurately, Titleist will only know what the received order was for, if not the intended order.

While the model is current, you can purchase an exact replacement or if a component fails, Titleist will replace it.

If at some point your swing is changing, you would need another fitting and if necessary you would need to adjust your current products or purchase new. Titleist won't be recommending new versions to you. It is impossible to predict the exact replacement model without a new fitting.

What you were asking about ongoing upgrades would be what a custom club maker would offer, not necessarily using OEM equipment from TM, Callie, png, or Titleist. In that case, the irons wouldn't be under $200 a piece - more like $300-400 a piece. There's off the rack from box retailer in sets, OEM equipment adjusted to you (Titleist's recommendation) and the full custom club experience. In my case, being LH and approaching 70, there is not one club on a store's rack that is ideal for me. Titleist prices per club, so I have full flexibility for my clubs. I only carry 5 irons but have several hybrids and fairway woods. That's why I get fit by Titleist or Titleist certified fitters. My bag works for me.

I did a fitting for my AP2’s when I bought them a few years ago and while it was a good experience, after an hour and going through all of the procedures, everything for me was dead stock. I would like to get fitted for a new driver, but wonder about the cost. A driver is $500. It part of optimizing it is finding the right shaft. Does that cost add to the initial driver cost? If so, then are you basically buying the stock shaft that you probably won’t be using?