Whether bypassing ad banners (e.g., by direct XML access) is "abuse" depends entirely on site policy. If a site exposes a public XML (or RSS, or whatever) interface, but not does qualify their use by a stated policy, then it's "anything goes".

If a site depends on ad banner money for revenue, and exposes direct XML access, and doesn't clearly state limitations on that access, then they're abusing themselves.

If you feel guilty about bypassing Monastery ad banners, there's always the Offering Plate.