if not you'll just be guessing and second guessing what you should do to increase performance and put money and time in the wrong area... I'd like to know if I cut holes in my lining if it actually made any difference, if I install titanium shims does it hinder heat travelling to my calipers, if I get a different type pad does it help heat buildup, if I install brake ducts does it do any difference... otherwise you just work on the 'feel' and think that something made a difference when it really didn't.
I see your point, but can you tell the difference between discs at 600c and discs at 700c? I can't, but I can tell the difference between fluid that's boiling and fluid that is fine, which is why feel is more important on brakes than raw data. Like at said, when you're racing then there is a good reason to use temps to gauge how the brakes are reacting over long and short distances, but for the average road or track day driver it's irrelevant data.
When I buy brakes I look at the numbers but also ask for experience in terms of feel, and if the setup turns out to be fine then it doesn't matter what temps they get to.
Ok lets say my discs are 500C and calipers get up to 200C so in theory that means my fluid more then likely cooked (due to the fact that it's probably likely to be at the wet boiling point). If I swap my discs for say DBA's 5000 and they actually help bringing down my temp on the disc to say 400, and the caliper only up to 150. Then I've saved my fade and cooking in one go. If I on the other hand notice that the replacing discs didn't make ANY difference on the temperatures I know that the problem is elsewhere and spend time to fix that. (and get cheaper discs instead). I know it's a bit rough but that's the idea I'm working around. So to me (even only doing trackdays) it's important to know WHY and address accordingly.