I've got a Nela sf 14, 18 and 14 refina sf the refina sf 3, half a dozen laying on trowels in different sizes 11-18 a speed skim 1200, sponge floats and plastic trowels and although I don't use them all every set and I could manage without most of them it makes my life so much easier, allows me to stay interested and try new techniques and means I'm prepared for any situation and can chose the trowel I know will preform best for the background, material and environment because let's face it most jobs are different.
I agree it's a royal pain in the arse having so much gear but it makes me faster and able to tackle larger areas without having to panic.
If you can manage with a mt start to finish then good for you but why make the job harder than it needs to be?
Also I've seen a lot of old trowels turning out very average work using only MT (and some guys turning our awful work using only sf trowels I might add)
Personally I like to walk away from a job thinking I've left an exceptional finish for someone to eye up, people notice the difference between a nice uniform wall and greasy teard up nasty looking work even if it paints up the same some times.
I don't always get it right when trying out new techniques but it always looks better then work that's been laid on and finished with a steel trowel I reckon?!