what i really ask myself is, since the scremer cleans the amp, and shape the sounds, does it really matters if its a mesa, an Engl or framus kobra, or even bugera? since the screamer is what coloring the sound.
Of course it really matters, else I would like the sound and feel of Power- and Fireballz as long as they are boosted
TS does as you sais, it focuses the sound OF THE AMP a bit more. Using different types of Boost Pedals alters the sound less much then using the same boost on different heads ^^
I'm with TheWinterSnow, it's enterely possible to desing an amp that is tight on it's own but cleaning the input signal with a circuit like you'd do with a TS, and if it's switchable everyone is happy.
Dual Recto for example could need it, as I said.
But I think for more (punk)rock(pop)ish stuff you can go for a cool lower gain sound without a TS on that head, by playing with the crunch channel, channel and main volume. Will still be tight.
We metalguys just want it to be tight with lots of gain also.
Dual Recto is the only head I can think of that I consider a TS a must for that kind of music, most other heads I played also worked without one, at least live.
That's the thing that pisses me of more about the issue then the "expensive amp then boost it with 100$ piece", lot's more of technical error sources and switching work live with a TS.