Sure, ArcSDE (or whatever it's called these days) is pretty cool, but to be honest I've never seen it deployed in the wild on anything other than Oracle, and often in conjunction with Oracle Spatial. What I've heard is that while the technically support a bunch of databases, they're pretty in bed with Oracle and that is what it works best with.

Around 2007ish they pledged their loyalty to Microsoft and the Oracle users became the redheaded step children. I was at the user conference in 2008 and the few Oracle related demonstrations were poorly attended. Even my organization moved to SQL server around 2011. I left shortly before that.