Mainly coil life and possible health concerns. I popped in the frankly under performing ni200 head that came with my crown and figured I'd just use it until it was time to change. The stupid thing just won't die. Tank after tank and it just laughed at me. I pulled it after I got sick of the underwhelming performance, but it's still sitting here on my desk, good to go in a pinch. It's way harder to hurt a coil head in TC mod. If you never run tanks very low(say always refilling at the half way mark) or prefer vaping at a lower wattage the actual value of temperature control decreases, with the caveat that some people like extremely cool vapes and TC can reliably provide that with shorter ramp up time. I know a guy who only vapes fruit flavors at about 420 degrees and he loves temp control, because he can run in the 20-30 watt range for fast, instant heating without ever "burning the hell out of all of the flavor" as he insists most of us do, but for most people the most substantial benefit is coil longevity and health concerns.
The idea is, perfectly set, you're never heat damaging your wicking material, so the only thing that causes your coils to "age" is buildup on the coils themselves. Depending on the flavoring of of the juice in question, that can take a very long time. On the other hand, if you vape dark, sweet liquid that really caramelizes and gunks up coils, you might not notice any real change at all. There's also the health aspect, in that overheating your juice may produce some toxins that you are better off not inhaling as well as toxins from actually burning the wicking material and the "gunk" that builds up on the coils. I've had TC self built coils that lasted MONTHS before needing any maintenance and after burning off the gunk(which I hear you aren't supposed to do anymore, my bad) they were good to go again. I just used new wick, but frankly, I could have pulled the old wick back through, as it was still in fine shape. As for the health side, that's kind of a personal preference. I'm not too concerned about it, but some people take it very serious. I figure no longer smoking two packs a day and primarily breathing clean, country air gives me enough wiggle room on the occasional potentially harmful trace chemicals that I'm not terribly concerned, but your opinion may vary.
Compare that to regular, non TC mode, where you pick a power setting, and maybe that works great in optimal conditions, but maybe you get a little singing of the wicking over time when you hold the button down a little too soon or too late, and maybe the occasional dry hit if the wick isn't performing perfectly. Eventually it gets gunked up enough that you are "burning" the buildup on your coils just to get the liquid on top of that to vaporize and you can taste that, so time to change coils again. In a rebuildable the wick will actually just fall apart if you keep it in use long enough, from the constant minor singing and carbonization that occurs.
That said, on the Crown I really prefer the .25 Ohm heads at 60-70 watts in power mode, adjusted depending on the juice. The SS coils have non resistant legs soldered to them, so the SS TCR isn't exactly right, but even playing with custom TCR settings I can't get the things to perform as well in TC mode as I feel they should. They may only last me 2-3 weeks, but I'd rather get the vape I enjoy than a sub par temperature controlled one that lasts longer. Longevity at that point is just more time to feel disappointed.
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Will try this out tom at work and see if how it works out for me. Thank you for your input! 
