I don't really think that altcoins are having an impact on people that use bitcoin. And most people that use altcoins are people that use bitcoin. Bitcoin is more well known and more useful compared to altcoins. I know quite a few bitcoiners in real life, and only 1 of them invests in altcoins.
Some altcoins have special feature. For example, Monero (XMR) offers anonymous transaction. The transaction is untraceable. It is convenient for privacy.