Bitcoin is a currency solely because people are willing to put their faith in it as such.

Without Bitcoin's brand and trust it would not be as useful.

By supporting alt coins you dillute the Bitcoin brand as the ultimate currency. It confuses people who have never heard of crypto currencies and takes away from Bitcoin.

That being said, if everyone were using bitcoins I would support competition with other alternative currencies as at that point you are swapping good money for possibly better money. Currently we are trying to swap bad money for good money.

Bitcoin will create the infrastructure for alt currencies to survive. Focussing on that is more important than trying to bring another currency in to do that.