I'd say go. I am planning a trip for next May that will include my cousin's daughter and son. Alexa will be a month short of 4 and Connor (who hasn't been born yet) will be about 7 mos. We're still planning on going.
I'm an avid reader of UG and I know specifically which section you're talking about. While I understand that thier research shows that kids 6 and under report the hotel pool as their favorite part, that doesn't mean you shouldn't take them as long as you have your priorities straight.
UG also has UG to WDW with Kids that specifically says, "As concerns infants and todddlers, there are good reasons and bad reasons for vacationing at WDW. A good reason for taking your little one to WDW is that you want to go and there's no one available to care for your child during your absence. Philosphically, we are very much against putting your life (including your vacation) on hold until you children are older."
I've read both UG and UG with Kids and the general bottom line they seem to have is that kids under 6 won't remember or be able to understand most of the trip. That doesn't mean that you shouldn't go if you want to go or that the kids won't have fun while they're there (though probably a different kind of fun than an older kid). And if you usually hit the parks with force when you visit, you'll probably have to comprimise a lot by doing things made for little kids (like the various playgrounds) and taking lots of breaks when you're with little ones.
I don't think that UG is being overly negative in regards to this issue, just realistic. UG, as are all guidebooks, is primarily for first timers. They just want to make sure that people understand that if you're going with young kids, you have to put them first. If parents aren't willing to do that, it'd probably be better to wait until the kids are older and can (at least in some manner) take care of themselves (ie feed themselves, go to the bathroom alone, handle the parks without a stoller, etc).
Personally, I was 13 and my brother was 10 the first time our parents took us and I don't begrudge my parents at all for waiting. I lived in Germany from age 4-7 and barely remember it at all. If my parents had taken me to WDW before then, I'd have absolutely no recollection of it.
I have no concerns about taking Alexa next year because 1) she's tall for her age and will be able to get on most rides, 2) she's pretty laid back and doesn't scare easily, 3) my other cousin and his wife take her to water parks and Busch Gardens in VA frequently enough that she understands the gist of amusement parks, and 4) there will be 12 adults (the youngest - my brother - will be 20 and the oldest will be her 51 year old grandfather) to watch after her and her brother. IMO, those are the kinds of things that should be taken into consideration. Overall, this trip is for those 12 adults with the added bonus of getting to watch Alexa's eyes light up at the sights.
