If you rise the fee you are making it more difficult for people to go into Bitcoin.

All this conspiracy about moving up and down the market its fun, but take it with distance. People that try to move the price sometimes win and sometimes looses. If it were as easy as you say everybody would be doing it.

There is a slight risk that some other party will take the other side of the trade because the exchange computers perform the order matching.  Thus, fooling around with dummy trades in a trending market could expose the dummy trader to the trend - its not entirely risk neutral.  So I expect that such dummy traders have been burned in the recent Mt Gox downdraft and either liquidated - if thats where they want to be, or holding current positions - if thats where they want to be.  To put it another way, by conducting dummy trades that have a chance of non-dummy fills, the dummy trader takes the unintended risk of real trading.

Actual market markers are not trading with themselves and their market-making behavior is more efficient when there are zero commissions, as is the case with the conventional FOREX (foreign exchange) market.  I applaud the lower and zero commissions at Mt Gox.

Without fees, mtgox wouldn't have been able to afford to replace what was stolen in the hack. I think fees are virtual insurance for if anything goes wrong, too.