Why wouldn't there be a need?
Once generation of new coins tapers out, there will have to be sizable transaction fees assessed each block, to ensure there isn't a reduction in security of the network. Once bitcoin goes mainstream, there will be orders of magnitude more transactions being processed per-block, and a commensurate increase in the size of the blockchain.
I've played around with some economic models of what per-block transaction fees would be needed to maintain the security of a mature, stable monetary base currency and avoid a tragedy of the commons. Working backwards from those goals and the network parameters, one can arrive at an estimate of the minimum necessary per-tx/per-kB transaction fees. The network parameters are continuously changing, however, so occasional re-adjustments would be necessary, just as is the case with the difficulty parameter.
The bitcoin developers are continuing to tweak the transaction fee parameters and rules with each release. Automating this process would not only make the network safer, it'd take a lot of the heat off the backs of the core developers making these unpopular choices.