Now, THAT's an awful idea. Finally someone is thinking about us poor Germans ;-) This is the greatest thing since sliding for umlauts.
My main use of this new feature would be exactly like in Jean's example: Getting an uppercase "Sch" (like in my name). I un-learned using the shift key since sliding is so much easier. Consequently I don't write "Sch" as shift-slide-s but as slide-s for "S" (as I'm on the s-key before realizing the need for shift) and tapping "ch" afterwards.
As most english and american writing people will consider this an obscure and esoteric feature I thought about other uses. I think there will be many cases of grouping two slides onto one key. Imagine a macro which includes a timestamp while the "long" version includes a date'n'time stamp. Or you build a custom slide for expanding some text, using the "long" version for the plural form. Lots of possibilities - without losing much.
Implement it!, I'd say ;-)