While I personally find these question useful, I see that in some cases they may open the floodgates and that wouldn't be a good direction for the site to head in.
For example, I answered a question about BBM as it seems like something an owner (or potential owner) would be interested in. That question also specified a specific phone, so it would be interesting if the app was only available in the Nokia Collection. I would also think a similar question about Instagram would be OK—especially since an official client is now out. These apps can be deal breakers to users, so I think they carry some weight (although an internet search will normally give you an answer in the first hit). In saying that, it would also be a problem if it turned into hundreds of questions like "Is APP X available on Windows Phone?" and the answers typically result in a yes/no question.
It is a fine line, but I would say questions like "What app can send free SMS over the internet?" or "What app can apply retro filters to photos?" would not be OK. Those question would normally involve a lot of personal opinions about which ones are best.
So after all of that, I would say, allow it for questions about popular apps, but monitor the situation and shut it down if it starts becoming a problem. Since we are still in beta I assume we are allowed to see what works and what doesn't work.
EDIT: I have changed my opinion on this. As stated above, it doesn't take a lot of effort to find out if an official app is available or not, so I think they should be off-topic. Since this answer already has an upvote I will leave it as it is.