Algae questions

To all you algae experts out there,

I have been reading this list for a while and don't remember if anyone
has discussed this before.  It also seems to me that the FAQ assumed 
algae was bad and needed to be gotten rid of.

Is there some particular reason why we
shouldn't allow algae to grow in our fish/plant tanks?  Do the reasons
differ depending on what type of fish/plants you have?  Or do they differ
depending on what type of algae is growing?  Is the main
reason purely aesthetics?  Doesn't some algae help the fish/plants to
be more healthy?