Well of course Maiden are probably THE most successful classic heavy metal band in the world today - I'm not counting Metallica here, cause Metallica have changed/adapted their style throughout their career, whereas the Maiden sound has remained the same basically from the beginning - whether this is a bad thing or a good thing definitely lies in the eye of the beholder.
I guess for most of the die hard Maiden fans, they just wouldn't want to sound "their" band in any way different than what they are used to from their early stuff. But I agree - many of the songs sound really very much alike; the main reason for this I guess is because Steve Harris writes about 90% of the songs, and has been doing so ever since the band came into being.
But you gotta give Maiden more credit than their mascot Eddie, in any case - simply by putting out "The Number Of The Beast", they released an album that not only influenced a whole generation, but an entire genre - everything that is Power Metal today ultimately goes back in some form or another to that album. And of course they also had some other great records and songs back in those days. And even if the song structures have become a little predictable over time - well, that is the case with many bands, I guess it just shows more with Maiden since they've got such a characteristic sound and songwriting, and since they've released so many albums over the course of the years.
But I agree with you - Maiden live are just awesome, I've seen them twice thus far (first time back in 2003 on the "Give me Ed..." Tour, and last year at Wacken), and both shows were awesome. But really - Eddie is just the icing on the cake; the status Maiden achieved especially with their early works is definitely earned, and besides - they've now released some quite decent albums again as well since Bruce Dickinson's return to the band... so considering that Maiden are still alive after so many years, still putting out pretty good records while having always maintained their style - I guess I'd consider it more of a "pro" for Maiden instead of a "contra", heh.