Wouldn't be advised really. In general, any server shouldn't run anything that isn't neccessary to minimise possible security risks. As packages like Apache + Samba are regularly updated to patch bugs + security issues, you wouldn't really want them running on a DC unless you want to be patching an awful lot. Even so, with all your user data + files stored on your DC, is it worth the risk having these possibly available through a service backdoor? Also, look at performance. If you're setting up a Samba server to act as a DC that is going to handle a lot of concurrent users, and your webserver is likely to be receiving a lot of hits, you don't want one impacting on the other.
To be honest, you don't need a very powerful machine to run a web + ftp server so you could easily install this onto another machine that needn't be expensive with minimal hardware, leaving your DC on the better machine and able to concentrate on maintaining them separately.