I crosstie simply because of how my barn is set up. The horse stands pretty much smack dab in the middle. Gives me plenty of room all around him to work. I could single tie him, but then I'd have to walk around his hind end to get to the other side, or walk under his neck. I like to have lots of room to work, and I'm just not as eager to be bending and walking under necks as I used to.
You can also ground tie which eliminates having to duck under the neck if you've ground tied in an open area, but there's a greater risk that must be taken then, particularly if it's a busy barn. The horse has to be better trained in this instance.
A lot to times I don't even attach the crossties. The horse just stands there between them.