I can see the discussion here but i would say that altcoin help Bitcoin a lot in terms of New users being introduced to bitcoin. I also believe that altcoin are not used only for trading or so we say useful. But instead altcoin are somehow a testing grounds for some possible update to bitcoin in terms of application with regards to masses' reaction.
If we check the forum activity we can see that the most popular board is altcoins so I understand that many people were/are introduced to cryptocurrencies through these altcoins and learn more how these work and learn the power and the potential of bitcoin.