what a lot of people fail to understand is that mining is a cost of bitcoin. it may be a necessary cost, but someone in the system has to pay for it. it is unclear in future whether the costs of bitcoin's mandated proof of work will outweigh the benefits of the currency. for example, the costs may be so great that the transaction fees that need to be paid to miners will be greater than the transaction fess of paypal or mastercard. that future comparison is, among the developers, an open question.
thats a little OT but an interesting question. if the official client makes no default minimum of a transaction fee and keeps relaying all transactions that means everyone sending a transaciton decides what transaction fee he pays. miners might reject some transactions, but only if there are enough other transactions that do have a high transaction fee.
eventually that will be the only income of a miner, and by mining the miner makes it less likely that the bitcoin network can be attacked, so he is a benefit to the entire network.
but network security isn't a benefit to the 1 guy who is right now sending a transaction. so why would there be any significant transaction fees as a whole?
why doesnt the mining community dry up and a large number of transactions are secured by very miniscule computing power?
I guess(!) initially the network was supposed to be secured by each client doing some mining and specialized hardware wasnt anticipated. correct?