Is there any reason they are unpopular other than they don't have much momentum and it kind of sucks to type? I think they are cheap domains but have avoided them for the assumption that they just don't get the SEO of a .com
Not being judgmental. :) I've started a few sites and I always think about stuff like this, especially with e.g. IO domains being subject to reclamation by the countries that manage the TLDs. Seems like MOBI is a bit more secure being backed by Google + Samsung and a few others though