Maximo has a few good criticisms of altcoin. The biggest one in my eyes is that bitcoin will absorb all altcoin technology into itself. My argument against this is:
While bitcoin may be able to absorb altcoins in the future, it cannot do it right now. Which means altcoins have value right now. By the time bitcoin starts to absorb altcoins, those coins will have huge network effects and bitcoin will have to fight it. Much like how altcoins had to fight bitcoins network effect on the way up.