First, to quote Cheap Trick (just because it amuses me to), reputation is a fragile thing. If I do a really cool groom's cake, and the wedding cake is less than excellent, I want no part of people thinking that I did both cakes. People are likely to say "Wow, who made that cake?" about a good one, then not say anything about the bad one in order to be polite. The most likely assumption would be that one person made both cakes. Not good.
Before you think "Oh, that wouldn't happen, come on..." Let me tell you about the dog cake incident. About 5 years ago, I did an extremely cute dog groom's cake. I didn't do the wedding cake, though, because I had too many orders that day to do both, and the bride chose the groom's over the wedding cake for me to do. She got the wedding cake somewhere else.
I arrived with the groom's cake, and the bridesmaids were waiting for me. They said that the person who had delivered the wedding cake wasn't a decorator, and couldn't "fix it," and could I try to? When I saw the cake I knew that there wasn't anything I could do to help them. It was just ugly...The color was totally wrong, the tiers weren't centered right, and the piping looked really amateurish. I told them that I couldn't do anything to fix any of that, because they just weren't things that I could fix, and also because it wasn't my cake and I didn't know how it had been constructed. I had to leave the dog cake there, sitting next to the ugly cake. Poor little puppy.
I talked to the bride later, and she said that the dog cake had redeemed the wedding cake to a certain extent, but that made me think about other situations that might arise. I decided that guilt by association wasn't something I was interested in.
(That wasn't the only ugly wedding cake I'd ever seen at reception sites. There have been plenty of cakes that I've seen that were horrifyingly bad, but luckily I had nothing to do with those weddings. You just feel bad for the bride in a situation like that, but if you have no groom's cake involved it isn't your issue.)
Other things that could come up are liability issues such as someone getting sick from the other cake and blaming my cake. Sure, that's unlikely, but if I don't know where the other cake came from, it could be from an unlicensed, uninspected bakery, so how would I know if they practice good hygiene while baking?
If I haven't been hired to do the wedding cake, the only time I'll do a groom's cake is if it's for the rehearsal dinner, and only if I deliver it myself to the venue. If that means missing out on a few cake bookings, that's fine, there are things that are more important to me than money. My reputation as a decorator is one of those things.