It helps you make sure you're on the right family line, for one. Members of my own family pitched in to have my uncle tested. We have two different lines of Millers in the family, as well as Brown, Braun, Graham and so forth. Lots of really common names. We've also had a lot of lies in the family about where it came from (i.e. my grandfather was German, but fought for the US in WWII so he told people he was Irish, grandmother was Lakota but told everyone Cherokee since they were on better terms with the white government). Some members of my family are really into genealogy, so they had the test done once they were relatively certain they'd found the right lines. It did also lead to some living cousins, which was a cool side effect, and let us know that one line had followed off of the wrong person from a small village where two people had the same name.
As for cons -- the biggest one we thought of was in cases where parentage had also been lied about. A child conceived with a mistress may be taken into the family as belonging to the wife, a father may not always be the man to whom the woman is wed, etc. Luckily, this didn't seem to trip up our search too much, but could definitely be an issue. Other than that...none noted, so far.