In the olden days of NT4 domains people did do this - the primary issue they had to work around was that every so often (20 days or so, from memory) every machine renegotiated a new machine account password. Although it was possible to reduce the frequency that happened, it was never really an ideal situation and with Active Directory it's even less ideal.
It would be fine to use Ghost imaging as a disaster recovery tool to help you recover much faster from a failure, but it's really not suitable for the domain database. Of course, provided you have several replica domain controllers - which is a good idea - then you can rely more on the domain replication for that aspect.
Even so, for most companies I'd suggest backup for your DC's is like any other critical database and it's worth going for a strategy that gives you defense in depth. Ghost can accelerate your use of other recovery tools and it should give you a worst-case baseline to fall back on, but I wouldn't recommend you use it as the primary backup technique even if you do have replicated DCs.