Orphaned blocks are wasted having power, so the network becomes less secure
For more detail, I'll try to present a few numbers.
I referred to the bitcoinmagazine article:
http://bitcoinmagazine.com/8972/quarkcoin-noble-intentions-wrong-approach/Assuming it takes about twelve seconds for the block to propagate through the network...
Bitcoin (10min): 2% work wasted = 2% less secure
Litecoin (2.5min): 8% work wasted = 8% less secure
Primecoin (1min): 20% work wasted = 20% less secure (than all of the work dedicated).