First, let me state that I agree with what Willie has written: it is not nice at all to make such a suggestion about cstheory on MSO, neither here. If you want to discuss cstheory you should post on its meta not in other places. Moreover, you are not an active user on either of these sites so I am not sure you are qualified to express opinion on the issue. It is better to leave these issues to people who are actively using these sites.
Second, remember that an SE site is not defined by its topic but by its target community. We have argued about this kind of issues several times with people who are not active participants (e.g. shouldn't we merge MSE and MO?) and I should say I really don't understand why these people who are not actively participating on these sites feel confident and knowledgeable enough to make such major suggestions. It is amazing that almost always these suggestions come from people who are not active on these sites which is kind of annoying, even more so when they know that they shouldn't (as you did).
Now, these said, as a user (not a moderator on cstheory) I think there is a considerable overlap between MO and cstheory, and many theoretical computer scientists definitely feel that they are also mathematicians. But many theoretical computer scientists also definitely feel that they are computer scientists.
There is a considerable number of questions on cstheory that might not be very welcome on MO. Note that we interpret TCS in a broad sense:
TCS covers a wide variety of topics including algorithms, data structures, computational complexity, parallel and distributed computation, probabilistic computation, quantum computation, automata theory, information theory, cryptography, program semantics and verification, machine learning, computational biology, computational economics, computational geometry, and computational number theory and algebra. Work in this field is often distinguished by its emphasis on mathematical technique and rigor.
I personally don't have a strong opinion either way right now, I haven't thought about cons and pros. I also don't know how cstheory community would feel about this. It might be the case that cstheory community would be fine with a merge, so far we haven't felt a need to discuss it and right now I don't see why we need to discuss it other than to make the world fit into someone's mental picture who is not an active user on either site. If a user who is actively using cstheory suggests it and explains why a merge with MO can be beneficial for the cstheory community then we can discuss its cons and pros (on cstheory). Until then I don't see a reason to discuss this further.