It makes it a no-no because of the idiots that are aligned with a party, merely because they are aligned to that party. Some people would never forgive (and ergo vote for them again) Labour if Miliband allied with "the party of Maggie Thatcher" and some Tories would never forgive being allied to "them Communists who want to bankrupt the Country". Unfortunately voters who vote for a party just because their parents did and are apathetic to the parties' policies are a massive portion of the electorate.
The truth is that the difference between the Tories, Lib Dems and Labour makes no real difference to the normal man.
Most people don't like to hear it but if you vote Labour in a couple of weeks, you're a Tory. If you vote Tory, you're a Labour supporter. This is obvious as Labour are always chasing the vote of the c. 8 million Tories in this Country and their policies clearly show this and vice versa.
The difference might not be as much as many of us want it to be but it still exists. Labour do a much better job with the NHS and I don't want to vote for a party that will either pull us out of the EU or set a referendum.