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.
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.