yes perhaps you could when you cut through the two layers of tube, break the chain, and release bolts... but thankfully the majority of riders aren't thieves, and the majority of thieves don't carry chain breakers.

Saying that, there is a brooks and bike theft epidemic in Toronto. I have heard of people getting their chained saddle stolen, but that is when it is left out overnight. I wish I had written my name on the underside of my brooks I might be able to find it now.... http://www.thestar.com/News/GTA/article/464768