Good bloggers are good allies, bad bloggers are bad allies. I've seen and know both. The first one is...
I've noticed quite a few who specify in their review policies that they will NOT review a book they don't like but will instead pass the book on to another blogger they think might like it.
Yep. They can be considered as good ones and good allies (Regardless I love if someone is pointing out my mistakes.). I know few and used to read their blogs too, because they're fair and balanced.
And there are the bad ones whose can't be neutral as a good critique should be, but they're rather ridicule anything and anyone, just to express their inferiority complex, the hate what lies in them, or they're just writing something bad, because they feel themselves so important if they can kill the work of someone without any true reason. Now these are the bad ones. Unfortunately the last ones are in greater numbers.
The general problem is, most self-appointed critiques want to force their view on others. If they don't like something, you must hate that too. If they like something what is an ultimate crap, you must like it too or you're not welcomed. But if these people should tell something face to face, they would just say some false, lame excuses for their critiques.
There are professional, well respected critiques, who have the background to do this job (As being a critique is a job, which requires talent and knowledge.). But nowadays, if someone read a novel, that one already believe he can write a better one (Some people already declares themselves as writers, because they're writing few lines in a blog.). If someone is watching a movie, that one already believes he knows how to create a better movie. So nowadays everyone believes they know everything, while they know nothing in the reality. That's the problem.