Voting based purely on political affiliation is pretty @#%^ing stupid, too.
Mmm, maybe on a local level. On a national level it makes all kinds of sense, as the party should reflect your stance on most of the issues that matter to you, and it's the party that can bring pressure on its own members. If you elect an idiot of your affiliation, there's at least a chance that he/she can be brought to heel on most issues that matter.
When a whole bloc of idiots gets elected at once, it's harder, as our good friends the Republicans are finding now.