First: the number of coins mined per 10 minutes are more or less fixed. It doesn't matter how many miners are hashing: currently there will only be 25 coins each 10 minutes. And each 4 years this will half (so in 2012, you got 50 coins, in 2016 the miners will only get 12.5 coins). This is build into the protocol (crypto currencies have a deflation built in).
To incentivize miners to keep mining, it is essential that it becomes a mainstream currency (not a hoarded commodity) with a huge number of transactions per second. Although Bitcoin transactions are nearly free, if there are hundreds per second, it will count up for miners to be profitable. But here I have some doubts as well (to be frank: I think they will change the protocol and higher fees if the incentives aren't meting Miners demands).
And yes: Miners are essential for maintain security of the transaction process. The more the better and the more decentralized the safer.
