The '09 MP has the SATA slots soldered directly to the logic board, so an internal RAID is only possible by software (OS X), or Apple's RAID card. 4 drives may also be a limiting factor as well.
So the only other option is 3rd party. But to use it, you
have to go external. Quite possible with existing products, but more expensive by a considerable margin.
By having the possibility of an internal array, you can start small using the internal HDD bays, and not have to purchase an external enclosure. It was even possible to expand to a max of 8x 3.5" drives internally, provided you placed the optical drive in an external enclosure (not that expensive, say $100USD). The possibility of growing over time can be
very attractive.
Now, that type of growth/initial low cost is effectively gone if you require/desire a 3rd party solution.

Apple and their proprietary nature strike again.
Well, at least it's not as expensive as the FB-DIMM's were initially.
If the '09 MP had a greater number of DIMM slots, it would be and less expensive to upgrade using 2GB sticks (Unbuffered), and easier to upgrade over time. (Registered, if needed, would be more expensive of course).
Unfortunately, the layout Apple used wouldn't allow it. There just wasn't enough room to fit any more.