Where Ron Paul is eligible for write-in you should sure as hell write him in. Where a write-in for Ron Paul would get counted the same as a vote for Mickey Mouse, write it in if you are voting your principles but are not concerned with your vote having as big of an impact.
Sure, no one wants to vote for the Republicrat Obamney. If your vote doesn't have a chance of having an impact, it may not be wasted just misdirected in that case. An Obama/Paul vote in a Romney state won't mean anything, just like a Romney/Paul Vote in an Obama State won't either. It would only be viable in a toss-up state, and even then it wouldn't have that big of an impact unless the # of write-ins becomes a torrent.
But think of this - A vote for Gary Johnson on the other hand would get the Libertarian party closer to the 5.000% popular vote threshold to be eligible for public funding from the FEC. This could potentially give the liberty movement a leg up in 2016 as far as ballot access, reach for voter education, and simply visibility.
by the way, 15% is only in relation to national polling. A candidate has to be polling >15% to be considered for inclusion in a national debate.
Pride and greed hold us all back, to the benefit of the few and detriment of the whole.