The most onerous part of GFDL is the part about listing credits, and the GFDL itself is supposed to be included in each text
(even Wikipedia doesn't technically comply with that part, it's a link) Technically, articles copied here from Wikimedia projects are in violation of that unless the list of contributors is grabbed from their edit history and added here too (perhaps as a comment). In practice, Wikimedia are happy with a link back to them and the original article, but it's understood that they're playing loose with their own license terms by doing that (this business where they have trouble following the terms of their own license is the main reason there are sometimes regrets about choosing it). Another problem with GFDL material is that it can't be incorporated into CC works unless all authors involved in that part of the work also agree to a CC-compatible license (which is why I do that). At this writing, the iBook article at guides.macrumors.com is violating that license (and I'll fix that, since I'm bringing it up); it's a pain.
The CC licenses are a bit less weird. For example,
CC Attribution is short and simple; only requires noting what license and authors are involved, with no other restrictions. It's a low-hassle license all around, pretty much the same as the MIT or new-style BSD. The one major exception is GFDL projects; they can be wary of mixing even if in practice things will be hunky dory.
That's where the multiple licenses come in handy. Each other project can pick the "open" license that's compatible with what it is doing, the authors are entitled to credit under any of the options (and each project can decide how much red tape it wants to handle).
