The answer is weird and confusing, so I'll try to explain.
First, I misspoke that it's not installed. XCache IS installed, however it's not really set up to be the OpCache that W3TC wants. On the server xcahce IS supported, however the way W3TC works with xcache does not work well (if at all) with our implementation. So when someone says "When I try to use Opcode: Xcache in W3TC, I receive an error." the answer there is "Yes, we know, it doesn't work the way W3TC wants it to." So what I shold have said, and again, my bad, is that "Xcache is not compatible with W3TC, and not supported for use with it."
But that's not really the heart of the question. See, we have MemcacheD on the servers for dreamPress, which we DO feel is a better mesh, and while you certainly can activate xcache with DreamPress, it won't harm anything, but it also won't magically work with W3TC, which again, we don't support on DreamPress right now.
The opcode caching we built in to DreamPress is evident in the file object-cache.php found in your wp-content folder, and that was built to use MemcacheD. If you wanted to use W3TC you'd have to remove that and THEN tell W3TC to use Memcached for OpCode caching, not Xcache.
Which at this point is all theory. I haven't tried to use W3TC on DreamPress in depth since we don't support it and I put more of my time into the Varnish plugin that I maintain the code for.