The problem with tactical voting is that you take the chance that your vote will be wasted, and you might be up for nomination, instead of someone esle.
eg if say you were going to vote TIm, but voted Jonny tactically, then Tim might not be nominated, BUT the one less vote he gets might mean you are nominated instead.
for tactical voting to work, you have to trust that other will do the same!, and I dont think the HM's trust each other enough