I am not sure why a T2 would need a good case and an S900 doesn't... You break one, you need a new S900. At $1600 roughly each. So you only have to break ONE S900, and you might as well have got a T2 in the first place!

The need for a backup drops dramatically if you a) get a good case, and b) get a better built, less toy-like keyboard in the first place...

Buttons, sliders, knobs, keybed, all need to work flawlessly if the arranger is of any use to a gigging pro. All of these have had anecdotal problems on PSR's (and considerably less on T2's) in situations where they are gigged a LOT. The insistence by those that use them for a backup is telling... You are essentially saying to gig an S900, you need TWO. So they don't cost $1600, they cost $3200. To be usable, that is.

Or you could just get a decent case, and be careful...