The reason they make larger hard drives with lower RPM is for people who are concerned with how "green" their PC is. They are usually marketed as a low-power alternative, because they consume less power rotating at a slower speed.
The slowest part of a computer is the hard drive. Making that slow part even slower will literally slow down every facet of using the computer. Using the internet will be slower, gaming will be slower, video editing will be slower, using MS Office will be slower, etc. Any time the disk is accessed, performance will be slower.
For the average computer user who doesn't know any better, they probably wouldn't notice any difference. Those who have had 7200RPM drives for a while and are used to that level of performance, the slower drive would be immediately apparent.
If the drive is used for infrequent storage or backup, then it would be okay. For everyday usage, I wouldn't recommend it. I would actually sacrifice capacity for greater RPMs -- go with a 1TB 7200RPM drive instead of a 2TB 5900RPM drive if you must. You can always add more capacity later.