Erm, the system is designed from the outset to have and use a HDD for games use, it's hardly the system's fault if the developers can't be arsed to use it optimally
If users wish to store tens of gigabytes of movies on the HDD at the expense of game storage, then the system is designed to easily swap out the internal drive for a higher capacity one, but to not bother to develop the game properly because of a tiny segment of users is well, dumb.
2-3GB's of the most commonly used data would completely omit it from being an issue, and out of even 20GB, it's quite a small amount.
The PC version of Oblivion is designed to be used with a HDD, the 360 version will take advantage of one if it can find one, both are just as likely to be storing lots of files on the HDD's, but somehow it's only an issue for PS3 owners (who as I said can simply replace the drive with a larger model if they so wish)?
Like I said, develop it properly and it's not an issue.
Ahh, but that makes it harder to needlessly moan about the PS3 doesn't it..