If Rubio runs, and becomes a major candidate, then at least some of the biographical coverage of him will focus on his Cuban heritage, which inevitably leads into a bit of coverage of the USA's screwed up Cuba policy. There's at least some chance that someone in Congress will try to embarrass Rubio and exploit splits within the GOP on the issue (Ryan, for example, was at least anti-embargo before being picked as Romney's running mate....his current stance is less clear) by proposing legislation to repeal the embargo and/or travel ban.
OTOH, if Rubio doesn't run, then none of this happens. The embargo and travel ban continue to be ignored issues for the vast majority of voters, and the status quo remains.
Why on earth would anyone want to piss off anyone in Florida? I could see someone thinking maybe Florida goes in the R column with Rubio on the ticket so there is no need to pander but people don't just stay pissed off for one or two election cycles. The Democrats could write off Florida for one election cycle but there is no way of doing it for just one cycle.
Secondly you would need to find out Rubio is definitely going to be on the ticket. That's going to happen pretty late in the game. It takes time to pass a piece of legislation as significant as lifting the embargo. You can't draft legislation and get it passed and signed into law in a couple of months.
I didn't explain my argument very well, so let me elaborate. This wouldn't be a partisan effort on the part of Democrats. It would be an effort by some members of both parties to go through with the repeal, which would happen because Rubio's candidacy would have elevated the issue. Some would see it as embarrassing to Rubio, but that wouldn't be the main reason for it coming up. The main reason would just be that Rubio running would give the issue a bit more attention, and that would make it more likely to come up for a vote in Congress.
Keep in mind, both Houses of Congress have already voted to rescind the travel ban (well, technically just to eliminate any enforcement of it, but that's effectively the same thing) back in 2003. The House has actually voted to rescind it on multiple occasions, but the Senate joined them in 2003:
http://www.nytimes.com/2003/10/24/politics/24CUBA.htmlAnd this happened at a time when the GOP controlled both houses of Congress. The amendment that would do this never actually got to Pres. Bush, because Tom DeLay had it stripped out when it was in conference committee, in order to avoid embarrassing Bush by having him go through with a veto.
40 Senate Democrats voted to life the travel ban. They apparently weren't worried about "losing Florida forever". And they were joined by about a third of the GOP caucus. Since then, the increased influence of Tea Party style libertarian impulses in the GOP would probably lead to yet more support in the GOP for repeal. (And of course, leading travel ban / embargo opponent Jeff Flake is now a US Senator, where he'll have a larger public platform than in the House.)
The only reason nothing's happening on this is because the issue is invisible. Hardly anyone cares about it. I'm just saying that if Rubio runs, at least a little more newspaper ink will get spilled on the topic, which increases the odds of something happening in Congress on it. Not necessarily increasing it to over 50%. Just increasing it above what it would have been if Rubio didn't run.
And this doesn't require Rubio to be on the ticket. Just run, and be a major enough candidate that a few more stories are written on Cuba.