I've read before that astrology can actually be a good indicator of personality IF the person actually believes in astrology in the first place. It becomes a sort of self-fulfilling prophecy where the person models their personality after whatever they've been told they're supposed to be like. Astrology seems to just use alot of universal personality traits that probably anybody could manifest at some point in their life. So then a person who believes in it probably internalizes a few specific traits as "their" personality and after awhile just turn to that trait automatically.

No idea how true it actually is, but it seemed to make some amount of sense. It could potentially be useful if you're dealing with a person who believes in astrology, but it's such a random thing that it'd be just as useful to study Myers-Briggs personality types.