No, not really. In an efficient marked risks uncorrelated with those of other securities shouldn't be compensated, so you should easily be able to screw yourself over by not diversifying.
But isn't the risk of diversifying compensated by a corresponding possibility of large reward if the sector outperforms? I wouldn't consider a strategy that produces modest losses with high probability but large gains with low probability sufficient to disprove my claim.
Here's the new thread for posting quotes, with the usual rules: