Trouble is the more alt-currencies we have, the more "pointless" mining we have, and we do live in a resource-starved world. Of course its a free market, so people can create as many currencies as they want ......... just gets completely pointless after a while, crazy even, especially if the currency has nothing new to offer the world.
Currencies with a fixed supply actually make people save natural resources by delaying purchases of goods produced using those natural resources. If the money supply is not fixed like with fiat currencies - people go into debt to purchase stuff they don't need, because they know that their fiat currency will be worth less next year, and thus they make producers produce more and waste more natural resources. If the money supply is fixed, people try to save that money hoping it will appreciate more next year, not spend it on useless stuff.