Q: Do linguists ever allow themselves to make value judgments when comparing one language to another? If so, what criteria are used?
A: To make a value judgment as between languages is implicitly to rate one speaker community over another: this is not a scientific stance, although it has attractions to many, since human beings are constantly striving to assert superiority over each other; and clearly different cultures (and indeed languages) attract different researchers personally for all manner of reasons—aesthetic, ethical, political, economic. But without preferring one community to another, it is possible to trace language histories, and see that some language communities have been more effective than others in effecting the security and spread of their language. The criteria for this are the brute facts of survival and growth of numbers.
I don't have a very deep understanding of other languages, but I have come to admire German for having words for some very obscure ideas and situations. I also love the sound of French and the simplicity of Spanish.
