I have 3 drives (a DVD-ROM, a DVD-RW, and a CD-RW), but if I was building a new system, I would only have a single DVD-RW in it.
If you burn a TON of CDs/DVDs, you could benefit from two drives for burning on the fly, but if you burn that many things, you might as well get two matching DVD-RW's so you can also have the option of burning two copies at once.
There's always the convenience of being able to leave the FarCry DVD in one drive all the time, but that's not a practical reason to go buy another drive. You'd be better off using CD emulation software, which will make everything run faster and make all your games much more convenient. I mean, with hard drives at under $1 per GB, it's no big deal for me to leave a bunch of ISO's sitting on a drive.