I think the best reason to vote Obama is to avoid a Romney victory though.
Simply speaking in foreign policy terms, I don't think the US understands quite how much ill-will the world actually has towards the Republican party. The US is tough, but not so tough it can literally go it alone, and bar Israel there isn't a single one of its key allies that would like to see Romney in charge. I think that will also strongly hamper any efforts he makes towards deficit reduction - even in key trade partners such as Canada or the UK the GOP swings between bogeyman and laughing stock in the media. Dealing with Chinese currency manipulation is all well and good, but not realistically possible without European, Asian and South American allies backing international negotiations. And pretty much all of those are about as eager to support Romney as cut their own noses off.
The memory of the Bush years is fading a little, but it takes a long time to undo the memories of two costly US-led wars (one still continuing) both of which were unbelievably poorly planned, a lot of somewhat insulting foreign policy decisions (what do you mean, we need to ask before striking your country with missiles), severe human rights abuses such as at Guantanamo, etc etc. Whether these critiques are fair or unfair, they're the popular perception and I think they're going to make it impossible for a Romney administration to operate an effective foreign policy (particularly militarily). Romney has hardly broken from that bad legacy either - insulting Britain when they're your closest ally bodes decidedly ill for negotiating with much less friendly nations worldwide.
Any of Johnson, Obama, or probably even Stein could carry a certain amount of gravitas and goodwill on the world stage. Romney will find himself extremely lacking in it just when he - and the US economy - need it most.