The real problem comes when it is mixed too heavily with politics.
That does sometimes happen; but for every instance of that, there are just as many - probably more - where politics and science are at loggerheads.
You couldn't get a clearer example than with the UK's current drug policy. The government went to an expert neuropsychopharmacologist (try saying that when you're pissed) - Prof. David Nutt - for a scientific perspective on the use of recreational drugs. When he came back with hard evidence that the government didn't like (legal because massively revenue-raising substances such as alcohol and tobacco cause infinitely more debility, illness and death than illegal drugs have ever done; the death-risk of taking Ecstasy is statistically less than riding a horse), he was summarily sacked.
That was a political decision, not a scientific one. As one of the foremost experts in his particular field I think we can take it that Prof. Nutt knows what he's talking about when it comes to the science, but the government has a sociopolitical agenda to push of which he fell foul.