If it's helpful and good quality, it's fine.
If the answer that recommends a different distro is high quality and contributes in some significant way to the question, leave it alone. This may occasionally occur; sometimes one operating system really is far better suited to a particular task than some other operating system.
The hallmark of good, acceptable answers encouraging OS migration is, I think, that they explain exactly why they're recommending using a different OS instead. Such an answer should be self-conscious that it's recommending something the OP probably would rather not do, and should make up for this disadvantage by clearly demonstrating the utility in migrating to another OS.
In practice, there aren't too many good and helpful answers that recommend only to use another OS instead. But sometimes, especially for embedded or "legacy" applications, such answers are the best. If someone asks about how to install Ubuntu (or which version of it to use) on their SOHO router box or how to make a live floppy system, recommending another OS is a good idea; in that case, even an answer that recommends using another OS without saying which one might be acceptable, provided it explains what it is about Ubuntu that is unsuitable.
Similarly, someone might ask if Ubuntu is well suited to some task. Sometimes such a question is too broad or primarily opinion based; other times, it's quite answerable. In the latter situation, a "no" answer (with explanation) will sometimes be correct and appropriate.
This is not an attempt to list all the situations where "use that other OS instead" answers are reasonable and helpful. In practice, it's uncommon for such an answer to be good. But it can happen, and occasionally even does.
If it contributes nothing, doesn't answer the question asked, or is very low quality, flag it.
If the answer that recommends a different distro is low quality and doesn't contribute at all, and amounts to little more than "just maybe that other OS over there might work better for this," then it's no good. It's probably very low quality, or not an answer (to the question that was asked) at all.
Most people posting on Ask Ubuntu want a solution to enable them to use Ubuntu while overcoming some problem, or otherwise want to learn about Ubuntu. Suggestions to use a different OS instead, especially when the OP has not indicated being interested in migrating to another OS, are often unhelpful.
In other words, I believe we should judge answers by whether or not they contribute anything, rather than by whether or not they recommend some other distro.