I don't deny that XP makes some awesome maps. The problem you get is that, no matter the education, there are some people who just cannot map worth a damn. For those people, it's probably better to get involved with any other maker, because they have a chance of making their maps suck less. When those people get into XP, they don't know what they are doing and mess up, only to release an extremely noobish game.
I'm such a person (one who cannot map, that is, not necessarily a noob), but I have better luck with a simpler classic design such as VX and Ace than I do XP.
But I will agree that XP has the tools to make some great maps, as long as the designer can use the tools correctly.