I think the reason a lot of people don't like Can I Play With Madness is because they consider it too commercial sounding. I guess they would rather have Maiden come out with an album and have it flop than have a song on it that's accessable to everyone to promote the record. As long as the songwriting is good and with Maiden it usually is, there's no reason to be upset about a band wanting to write a song or two to be singles off each album.
It's odd, Wasted Years has easily the most commerical and catchy Maiden chorus of all time. Yet just about everybody loves that tune but they get on CIPWM for being too commercial.