They ain't that expensive...I shoot Hasselblad and use other Zeiss products...microscopes, sunglasses...etc.
Ya know, the SCA 390 is the cheapest part of my camera! He He...
Seriously, the SCA's may seem expensive, but the interchangability is great. I use 60s and 45s during the "main" shoot, then 40s for casual shooting and for keeping in my pack, and I'm gonna get 2 70s for sidelighting wirelessly (other than my Pocketwizards), but the adapter is the only piece that stays. Plus, when I get done paying 1000.00 for the 70s, all I gotta do is stick the "plain" hot shoe onto the strobes to use on my RB/RZ set.
Also, it's great when the other shooters working with you have Metz adapters too. Often, I "share" my strobes with them when their Sunpaks crap out during the shoot (common with 511, 522 and 611 units).