Heh heh.
UMD allows you to emulate
having specific class abilities - e.g., it allows you to fake having a spell on your class's spell list (and also having a spell list, sometimes!) in order to trigger a wand.
It does not allow you to actually
use those abilities. A bard, for instance, could make a UMD check to fake having Divine Power on his bard spell list in order to activate a wand of the same, but it would not let him take it as a spell known when leveling up.
That's why the original example is bad* - if the chalice worked for anyone who could channel positive energy, then you could fake having that ability and let the chalice do its thing. If, instead, you actually have to channel energy to get it to work, then you don't have that ability.
Consider a sorceror with a high UMD check and such a staff. Why would he ever expend his own spellslots in order to power it, when he could instead just make UMD attempts to do it "for free"? (Since, if a Rogue can do it, it obviously doesn't really require expending a spell slot, since the Rogue doesn't have any to expend.)
* I want to say that they got rid of the example in the 3.5 version or in the second printing, or something, but that was years ago and I cannot remember it off the top of my head. And, obviously, I am AFB at the moment.
