I see 3 concerns with defaulting to the C: automagically and without parental control.
1) SSD sizes when used as C: -- Sure, they'll grow, but SW will outpace them. Already (as noted above) many of us are forgoing 'hibernate' (say RAM size -32GB sir?) and disabling default paging to the SSD which should hold either really hot data or static data. SSD's are the WoRM (write occasionally, read all the time). I'm OK with a 120GB boot C: SSD and have about 65GB free even with SPro installed, but its a very new system and growing fast.
2) Speed of access -- This is marketing fluff, nuf said. With 16GB of RAM, even page file accesses to the HDD are rare. I'm still checking streaming of long audio stuf simultaneously, but I'm not expecting problems and have my fingers crossed. I can't believe that a SSD would help in any case.
3) BOOT DISK is for the OS! --- With this whole UEFI (I just got upgraded), I actually now get to spend time wondering how best to organize my system. Even M$oft advises us mere mortals to separate our data and 'programs' (not to mention all the add-ons like VSTs, etc). My perspective is a program like Sonar is data! It is really useful for manipulating other data, but it is not the OS needed to run the machine itself.
I keep my Boot disk small so I can back it up frequently and easily. Even just a BU to a fast HDD is ~20 min. The 500 GB partitions for my music stuff is overnight. So at that point, I probably BU all the other partitions (~2TB) so thats another couple days. It needed to get done, but not on some timeline based on Sonar releases. So, the C: gets a weekly update from a stable system to a SSD and a HDD. I don't need to really re-write a bunch of small samples and even worse the dir structures associated with lots of tiny files, most of which I will never even peruse. These things belong in archives. RAM isn't an issue any more (for awhile at least).
I vote for lean and mean. I'm not aware of any advantage of putting the .exe on a SSD vs a HDD.
BUing a UEFI boot image is not quite as easy as it once was. I already have a 220GB SSD available as well and have set it up as an optional boot source. The C: drive gets lots of stuff dumped on it, cleaning is risky and really boring. I'm not BUing my C: because of Sonar but all the other 3 updates a day I already get to choose about and reluctantly accept. If I didn't put it on my C:, it has no business being there. If it can't be moved then that is an installation problem.