1. Obama is taking low interest independents and young voters who think he will end the war, bring big changes to Washington, etc.
2. It will raise his profile if Obama's campaign responds to any attacks. Remember, Obama called out Paul by name first, saying last week that "They don't poll Ron Paul, but I can beat him too."
3. He can gain good will among the Republican base by being the first candidate to really challenge the new democratic frontrunner as being a phony, empty suit, just nice words, etc. Most of the other candidates have spent months talking about Hillary. He can be the first to really take on Obama. The old days of Rudy-Hillary can be replaced by Paul-Obama.
4. Anything related to Obama now will get coverage. It's a long campaign season, and with 24 hour cable news, attacks by Paul on Obama will be brought up multiple times, raising Paul's name recognition.