I think the big fear is stasis. In each case you're put in a certain state of being without any recourse to get out of it, but wireheading seems to be like a state of living death.
I concur, but I think it wise to draw a distiction between wireheading as in an extreme example of a blissed out opiate haze, where one does nothing but feel content and so has no desire to acheve anything, and wireheading as in a state of strongly positive emotions where curisity, creativity etc remain intact. Yes, if a rat is given a choice it will keep on pressing the lever, but maybe a human would wedge the lever open and then go and continue with life as normal? To continue the drug analogy, some drugs leave people in a stupor, some make people socialable, some result in weird music. I would say the first type is certainly better then death, and the latter 'headonistic imperitive' wireheading sounds utopic.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.