I don't understand why Democrats are cheering for Tea Party candidates in safe GOP states - that will only add to the partisanship in Congress.
Several reasons: having a Tea Party candidate like McDaniel in Mississippi can change a safe GOP state into a not-quite-so-safe state, forcing national groups to spend money there at the very minimum, and occasionally letting the Dems win states they're not supposed to win (Indiana), but at a minimum having a Tea Party candidate gives them a national stage and can shift the national mood into a more anti-GOP direction (Akin).