People create trading bots for the same reason that people trade manually: For fun and profits. Bots don't "block a fair price", because what is a fair price anyway? Whatever people are willing to buy or sell for is a fair price. If the algorithm in a bot thinks 22 is a good price, then it doesn't matter that you were only willing to pay 21.

The only issue I personally have with bots on BTCT is the (too) small price increment (or 'tick') in orders. I can outbid someone by 1 satoshi per share and have my order executed first despite making what is effectively the same offer. This is already annoying when people do it manually, but bots can automate the process. This is one of the reasons I'm currently writing my own bot ^^