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.