The ICH10R is a decent chip, but still lacks the logic of a dedicated IOP, so for parity calculations it has to revert to the CPU, causing way lesser performance than a dedicated controller. In essence using the on-board chip is about similar to using a low cost Promise card. You get what you pay for. If you use the on-board chip, you will get mediocre performance, without the benefit of dedicated cache memory, dedicated parity calculations and BBM. Expect the sustained transfer rate of a on-board raid versus a raid on a dedicated controller in a raid5 to be around 50% with a much higher CPU load and see the performance loss of on-board to increase with more drives. Any more advanced raid configurations are impossible with on-board chips. You need a dedicated controller for raid 3, 6, 30, 50 or even more esoteric formats. The logic from dedicated controllers for staggered spin-up, spin-down, dynamic expansion, etcetera are major differences, as are setup parameters like stripe size, chunk size, block size and so on.

Not everybody needs a nVidia Quadro 2000 CX card, quite a lot of people are quite happy with on-board video for generic applications. However for HD/HDV editing or other more demanding applications, this may not be enough so they opt for dedicated video cards. The same applies to (SAS) disk controllers.