This is actually a common misconception, and you're wrong. If you read Satoshi's original paper, the probability of someone with substantially less than 50% of the total compute power decreases exponentially as the number of confirmations increases. The reason Bitcoin uses 6 confirmations is because this was judged to give the best balance between time taken to confirm and probability of a sub-50% attacker succeeding.

It has nothing to do with stopping attacks by someone with a majority of the total compute power - realistically, if requring someone to dedicate that amount of compute power for 12 minutes isn't enough to make an attack infeasable, making them dedicate it for 2 hours is unlikely to do the job either.

Edit: Also, requiring only one confirmation - which is what you'd have to do to make Bitcoin as fast as 6 confirmations in Primecoin - is especially risky because it allows any attacker capable of generating a block in a reasonable amount of time to double-spend with very high probability at the cost of one block's payout.

Yep. This is a mistake that a lot of Bitcoin-clones seem to make. Sure Litecoin "needs" less time to gain six confirmations, but you end up with a much lower ultimate difficulty. There's extensions on this with alt-coins, where people modify the client to have 10 second block targets and <10 confirmations, which ultimately makes the entire "currency" insecure and worthless.