I'm going to echo Mitch's point about costs. The biggest cost to going GUE is that you can't do it as incrementally--the jumps are usually bigger. I'll give examples from the tech curriculum since that's what I know:
Tech 1 - $1,750
vs.
AN/DP $800-900 + Rec triox $500-700: Less complete, not much less expensive
or
AN/DP + Trimix $1600-2000: Arguably more complete (usually 2-bottle), more expensive or the same
Note that I'm not counting the cost of Fundies, since frankly you should take that regardless of whether you intend to continue in the GUE curriculum. And the prices listed aren't the cheapest in the area--they're prices from the instructors whom I'd otherwise take the course from if I didn't decide to continue down the GUE path.
Equipment costs are negligible in this discussion--if you're diving caves, you're going to want to have a fairly similar setup. Honestly I think the minimalist philosophy saves money when compared to other tech setups...there's no need to buy the new hotness. Cave line is cheap, whereas all those fancy clips and retainers add up. No need to buy things with an H on them, although there are a few things that I won't buy from anyone else anymore after learning the hard way.
As far as why to go GUE, you should just read the Fundamentals of Better Diving by Jablonski (and add another con to your list...the founding cult leader has a funny name). Or read Peter's thread here for other people's reasons for starting down the path:
http://www.scubaboard.com/forums/dir/394339-why-did-you-take-plan-take-gue-fundies.html#post6028548
To me, it's a system where everything just makes sense. As I grow in the types of dives I do, I discover that someone had already thought of problems I didn't yet know I would have. For instance, when I took my OW training, I remember my instructors discussing the various ways that one could sling a pony bottle, and my impression was that for most people it takes a few dives and some practice to even start to get comfortable with it. By contrast, when I first clipped on an AL40, my reaction was, "Oh, that's what goes there." The space had always been clear on my left side, I just hadn't been putting anything there yet.
Finally, my understanding is that few people indeed take the "3" classes, and getting involved at some of the bigger projects is more about getting through Tech 1 or Cave 2 and doing some dives as a support diver.