Just imagine if all the colors that we see are one color such as black or white, of course we can't tell the difference anymore between a food or garbage. Surely it was not interesting, if bitcoin without altcoin just one color only.
Sorry really could not understand how your example also applicable for bitcoins. What you are going to differentiate by having more number of crypto currencies ?
But in my opinion too, having more crypto currencies definitely will be giving a common man some benefits like trading them and testing one new altcoins with some new concepts etc.
I think what he meant was if there was only bitcoins, we wouldn't appreciate it that much because we have nothing to compare it with. For example, if you only experience good things in life and no bad thing at all, you would think of all those good things as a 'normal' thing and not appreciate it at all. It's basically like the saying that goes "It takes sadness to know happiness." and all those ironic stuff.
He's right. If there was no altcoins, we wouldn't think of bitcoin as the best investment.