Jonathan, the one with the most active conscience, is also the one with the genuine magical talent. Warren, the tech guy, is also the truly evil guy, tying in to the general Jossverse suspicion of technology. Andrew's sort of in the middle, and, yes, demon-summoning does seem to be a slightly spearate thing in the Jossverse to other forms of magic. Note that in S7 we never see Andrew summon any magical entity for good purposes, even when it could have helped.