How useful is a second studio strobe?

I have an AB800 and an SB700 that I set up off-camera and trigger with the pop-up of my D90. I'm still in the experimenting/messing-around/learning stage, but I'm wondering: How useful do people find having a second studio strobe?

Right now I'm trying to decide between:

a. Getting an AB400 as a second studio strobe (and/or a backup)

b. Getting another speedlight as an additional off-camera slave

c. Getting a reflector or rigging up a diy reflector

d. Firmly telling myself that this is just Gear Acquisition Syndrome, and that none of the above would be worth my buying right now.