people that don't trust dollars

People Cryptoheads don't trust dollars. By and large, people outside the cryptohead / libertarian camp do trust dollars, and we want to try to attract those outsiders to the platform. (Whether they should trust dollars is another question which I won't go into here.) Dollars will help Steem get mass adoption.

We should have more options

The problem with having multiple options is that if any one of them has a massive rally relative to the Steem market cap, it'd affect all the other assets in the system. This seems like a hard problem.

BitShares solves this by having a different pool of shorters for each BitAsset. If gold rallies, then the most the BitGold longs can "win" is what the BitGold shorters have put up; they can't start tapping into the pool of BitUSD shorters, or the BTS that didn't short anything at all.

The way Steem is designed has all Steem / SP be the "short side" to SBD's "long side." If you had multiple currencies then you'd have to figure out how to divide up the "short side" among multiple "long sides." In BitShares there were a lot of supply/demand mismatch issues where a lot of BitAssets were very thinly traded, and even for popular BitAssets, often there were either too many, or not enough, on the short side. You'd have to make sure there's enough short side for each currency.

This is leaving aside the massive implementation problems, because in a great number of places, the code is written in a way that assumes there are exactly two liquid assets in the system, STEEM and SBD -- replacing this with a system that could support N assets would be a massive task in terms of the code that would need to be rewritten.