Futures would make the Bitcoin price much easier to manipulate.  One major banking institution or other wealthy group could manipulate BTC users in pretty much any way that they want.

Since BTC has less of a clear value than most other stocks and assets, it means that investors could be manipulated to push the price as high (or as low) as they want.

However, I suppose that introduction to a more mainstream and regulated audience could be helpful, as it could theoretically free us from the emotionally unstable investors we have now.