I don't really understand why people want "pure" characters.. other than for the only reason of calling it "pure," or for roleplaying reasons..
With a monk/(insert random secondary), you can be a "pure" monk build all you want, and just ignore your secondary. That is what I'm doing now.. but in case I want to switch it up a bit, I will always have the option of playing around with a secondary, and thus have twice as many skills to choose from. As has been already said, you don't lose anything from adding a secondary. You do lose possible opportunities for not choosing one, however.
Sure, Gaile mentioned that we will be able to change secondary professions later on.. but well, regardless, I'm still getting several skills for free in pre-searing by choosing a secondary, even if I won't ever use those skills.
You can choose to ignore the secondary and be as pure in your skills as you want, but otherwise, there's really no benefit at all to being one profession only.
But, to each his/her own.