Where the answer turns to 'no' is when the transmission has been neglected for years...fluid dark, thick, and smells burned. During normal operation, friction material sloughs off the bands and clutch packs into the fluid. Over time, the clutch packs get thinner due to the loss of this material, but the fluid is getting thicker which may allow the transmission to keep functioning. Damage is already done at that point anyway so might as well leave it alone.

The only 'seal' affected by changing the fluid is the pan gasket, and that has nothing to do with the transmissions operation.

A lot of Dodges (including mine) require band adjustments from time to time. Mine started shifting funny when cold UNTIL I serviced it and did the adjustments. I suspect a lot of shops don't look up what's supposed to be done and just change the fluid and filter no matter what. Neglect or incomplete servicing might be what's going on with your co-workers vehicle.

jmo

Now for the hard part. Many shops are struggling these days, and have been for years. Marketing experts, and business advisers have pushed the agenda, that in order to get to stay in business a shop has to push services. Part of that push is to allow a lesser trained technician to be able to perform the services, which reduces the labor cost to the shop, which results in a lower (overall price,arguably) for you the consumer. In everything like this there is always a break even point. Make the job too simple, and only hire someone who is only capable of such work, and eventually you find what you are seeing from your dealer. They have to push this kind of service to keep everyone busy. Go to a "quick lube" and you can magnify the glass ceiling that is there. They are not hiring career people, and from that perspective have no interest in getting them furthered training. I hope you can see how this is a problem in the long term.

Now back to your co-worker. Newton said, for every action, there is a reaction. Your co-workers statements are wrong, and make the first part of "the action". The reaction to outside, uninformed pressure that opinions like your co-worker shared helped to set the stage for everything that I wrote in the last couple of paragraphs. He is not only incorrect about how to service your transmission, his "opinion", which has been shared by many for years has had a profound negative effect in the auto service trade.

If you brought your Mustang here to my shop for servicing, first I would check with the manufactures recommendations to see what Ford says it needs. Then I would check the vehicle fluids myself to see if in fact it is really necessary. Now I have not seen a 2007 Mustang yet for anything but minimal servicing (my own BTW, a 2007 Mustang GT/CS Convertible) so off hand I cannot say if it has adjustable bands or not, although my expectation is that it does. If it was decided that servicing is required, the band adjustments would be part of the service. Remember it's impossible to tell if that was truly going to be needed without actually doing it. The pan would be dropped, and the filter replaced. A cooler line would be disconnected, and all of the fluid replaced. (No machine actually required, we figured out how to do this decades before anyone ever invented a machine for it) Are you ready for the funny part? I saw a bill for a Jeep Liberty from a national chain last week that does NOT adjust the bands when a transmission requires it, they used Dexron III, and dumped in an additive to assimilate Chrysler's ATF+4, and charged at least double what I do for the service! Too bad they didn't do any diagnostics, because that transmission had a legitimate problem that required it to be removed and repaired. (That's why it ended up here afterwards). Even though the owner felt there was nothing wrong with it prior to servicing, it was clear by the vehicles stored failure information that it had been experiencing troubles for the last three months. An uninformed perspective here, would fit your co-workers advice. By everything the owner could see "it had been fine until it was serviced". However that was simply because the transmission computer was compensating for trouble in the transmission front pump. The only part of this repair that potentially is a problem is the shop used Dexron III, and an additive of course instead of Chrysler's ATF+4. Now as a technician who has to repair the car, I have an unknown variable to consider because of Chrysler's stand that nothing except for their fluid is acceptable, and an aftermarket additive company is claiming otherwise. I now have to "guess" as to whether to trust the internal components beyond the front pump are still serviceable because of the fluid used, or play it safe and completely rebuild the transmission. No matter which I choose I'm the person that has to guarantee the transmission from this day forward.

To sum this all up. Check your owners manual first. Then go to a real tech, in a full shop and seek out his/her opinion. Remember that opinion is likely to be subjective, but still based on experience. Then decide for yourself. The longer that you want to keep your car, the more often servicing should be performed, and not just the transmission.