The Tea Party is as much about mobilizing the Republican base as it is their stated goal of "throwing out all the career politicians." (As an aside, I love that term. As though the best politicians would be ones who are just doing it in their spare time, or that their own candidates with $500,000+ in campaign financing aren't trying to make a career of this.)
And they've been very successful at doing that so far, successful enough to win primaries. The reason incumbents are usually the presumptive nominee is because your average voter doesn't get motivated by primaries. Very politically minded people and party hacks care about primaries. So the Tea Party has managed to rally enough support to overcome the early bird voters. That's something. (Not much of something, considering the abysmal nationwide turn out for primaries.)
On the other hand, they've also had pretty good success in governor's races, so it's not purely activating the most-likely to care part of the base.
Democrats will have plenty of ammunition to at least give moderate Republicans pause in November for voting. It will be a test of loyalty to the Republican party, whether to vote principle or party label. My money is on party label. Republicans will vote Republican rather than handing the democrats their vote by not voting or voting 3rd party, I think.
So that means the Tea Party will ultimately have accomplished its goal, getting its brand of Republican in power. Given the general dissatisfaction with government right now on both sides of the aisle, I think the Dems stand to lose more than people assume. Those hoping for Tea Party Republicans to create schisms with conservatives in November underestimate Republican solidarity in an election year.