Well in the first place it's cheaper; they can offer the PC to you at a lower price. And now, if you buy another system with another 5400 rpm drive you won't notice the difference. Just be prepared to wait a while when you're backing up gigs of data!
They are hardly any slower for sequential I/O. Maybe 10%; something you will never notice.
They are slower when it comes to latency/IOps though, but sequential I/O is very good for 5400rpm drives. Their data density is so high, they even beat 10.000rpm disks with Sequential I/O, even though they only spin half as fast the data density makes up for it.
So the prefered medium to store large bulky files on, is 5400rpm drives. 7200rpm+ drives are quickly becoming less useful and eventually will disappear.
Well for those of you thinking more rpms is better, what it really comes down to is the IOPS, and I have a web article to prove it. http://www.zdnet.com/blog/ou/how-higher-rpm-hard-drives...
i know there it's talking about 10K vs 15K but still, the concept is the same. By using that concept, quarter stroking your 5400 you should get better performance than say, your typical run of the mill 7200rpm.