I have to echo Kralizec's points. Pillaging provinces tends to paralise the AI and create massive stacks of rebels which I always end up having to fight myself for some reason...

Building in MTW is slow as it is, even rebuilding back up to keep level is annoying. Historically "slash and burn" tactics were used, examples being the "danegeld", but entire provinces and citadels were not razed to the ground. Landind in wessex and completely levelling London is just not realistic.