It becomes less profitable to mine, so probably less people will mine it. It reduces the potential amount of BTC that miners have to sell to recover their costs. The idea is that less selling will increase price.
Actually the transaction fees accounted for over 12% of the block reward for block 630000
Also, it may help to think about it this way: you don't mine the subsidy, you mine the transactions and reap the transaction fees. The subsidy, which halves ever 210000 blocks, is just that, a subsidy.
Imagine if more subsidies (like the US subsidy for corn that still exists since the first great depression) had been hard coded to expire?
