I just spoke with someone yesterday about this very topic. They were not happy with how Obama has conducted his presidency but they disliked Romney even more. They ended up voting for a 3rd party that had no shot winning. Their justification was that they were voting in a state that Obama was going to win regardless. Their vote was a "protest vote". I think many "protest votes" come in the form of not voting at all, no "blank", just not even leaving the house.
Wikipedia agrees with mk: In a two-round election, I guess you can always vote for your favorite in the first round and do a tactical vote in the second. Protest vote seems to have its advantages, though they seem long-term: (1) It will increase recognition of the party; (2) It will increase the respectability of the party; (3) It will increase the recognized viability of the party. I voted blank this round. I don't think I'll be doing it again.For example, in a simple plurality election, a voter might sometimes gain a "better" outcome by voting for a less preferred but more generally popular candidate.
Tactical voting makes sense in the first round already, as the French experienced during the presidential election in 2002: many people voted for their favourite in the first round, so many in fact that the traditional incumbent didn't make it to the second round ; leaving a choice between the corrupt and lazy president versus an old racist...