.mobi is just a brand just like all the other non .com extensions. you can use .com for a mobile site instead of .mobi, you can use .com for a non-profit site instead of .org, you can use .com for a video site instead of .tv, you can use .com for a regular site instead of .net, you can use .com for music sites instead of .fm, you can use .com for a british site instead of .co.uk. .com can be used for anything. no other extension is necessary. for a domain investment i think .mobi would be a horrible investment but to build a site on it would be fine for branding purposes. last.fm is very successful without the .fm extension being successful, no_url_shorteners is successful without the .ly extension being successful, etc.