Hmmm, yeah, my preference too would be to go for matched strobes.
Why go for an unmatched pair, rwe?
I was just trying to keep the bulk down a little; both for traveling and underwater. I tried using a Nikonos 105 paired with the Z-240 some years back when I first switched to digital and decided that my diving enjoyment was significantly less carrying that much around underwater (in TTL the slower 105 also tended to overexpose pretty often). But my present system is smaller so the difference between a Z-240 and S-2000 may not be that much.