For Fighters/Rogues, less so, I'm still not seeing the "downside" to taking a full path as a Fighter past the aforementioned build threshold.
My perspective is going to be different than yours, because I have played in a different culture game than you have (HQ) and probably also because I have been playing longer (24 years). So the comments in this post might not be entirely relevant to the game you play.
That said, I see a downside for all three fighter paths.
Defender - This path basically does one thing for all five levels: effectively increase your body total (and the body total of nearby friends). It does this in lots of different ways. However, in my experience, fighters rarely ever fall due to body. Most of the time, they fall due to one-shot disabling effects and with the exception of command reduction at 2nd level, this path provides no protection from those. 50 build spent here may move my fighter from "hard to kill" by damage to "nearly impossible to kill" by damage, but doesn't really protect me against my biggest weakness. If survival was my priority, I could probably get better benefit buying a celestial shrub (for access to spell shield scrolls), a parry or two (depending on how close I was before that 50 build), or maybe more racial resists if I am the right race (dwarf, wylderkin, selunari, dryad, etc.). And even after buying those things, I'd probably still have enough build to buy Hearty a bunch of times (basically mirroring some of the benefit of this path).
Ravager - I see lots of downsides with this path. First, it offers absolutely no defensive skills other than resolute (that is not meaningless). Second, the initial benefit is basically just a conditional prof and the secondary benefit offers a benefit that is almost completely wasted against 95%+ of NPCs (NPCs rarely have armor or wear armor). In fact, with the exception of the capstone skill (binding -> slow), I don't find any of these benefits to be particularly compelling. Berserk removes skill use, making it a mostly ineffective way to fight (though this is sometimes better than falling asleep or being dominated, depending on how readily allies have removal available) and usually there are enough blade spells to go around (and even when there aren't, you are a high enough level fighter to be near the front of that line). Even the meditate trick comes with the big downside that it puts Destruction, a universally nasty effect, on an ally. The majority of this path I feel could be mimicked with a few purchases of Hearty, a few levels of Craftsman (for daily silver to buy defensive potions / elixirs), a few critical attacks, or maybe even an earth shrub. And buying that way, even if not perfectly matching the path, avoids having to subject your allies to damage or debilitating effects.
Warleader - The biggest downside I see on warleader is that 60 seconds is basically a lifetime at the start of most battles. Sure, it can be handy just before walking into a module, but when the big wave battle hits the town with no warning, you and five other folks simply don't have the time to huddle in a circle for a minute while everyone else is fighting for their life. I think the level 1 active ability is absolutely, without a doubt, amazing. In fact, it is so good I might agree with your "no reason not to take it" theory (though it clearly comes with a downside), unless you really wanted another paragon path. But pretty much everything beyond that either has the 60 second delay or is an uninspiring benefit. For example, an instant meditate isn't really great when you had to spend 60 seconds doing something else to activate it. Similarly, resist voice is not something that comes up most games, nor is resist greater command (I can't count the number of times I have been hit by a greater command on one hand in over 20 years). Effectively what warleader does is turn the fighter into a very specialized mini-healer and I will acknowledge that isn't easily reproduced with build. However, unless you want to be a specialized mini-healer, that also isn't a very useful. And if you want to be a darn good healer, you could alternatively buy earth spells (possibly shifting to templar) or just buy craftsman and become the patron of a potion maker.
I want to be clear on something. I don't think any of the above paragon paths are bad. I think they all offer cool and very thematic benefits that will really jive with some characters. However, I don't think any of them are free of downsides and I definitely don't think any (or all) of them are must-buys compared to simply spending build on traditional options. For example, a focused bow fighter would probably look at these options and see nothing that disarms or breaks shields. And, as an archer, let me assure that taking shields out of the equation is critical. I think such a fighter would either look to a celestial shrub (disarm scrolls... YAY) or simply buy more fighter disarms, shatters, and stun limbs with that 50 build (maybe max out the meditate skill, too).
The nice thing about these paths is that I don't think any are complete worthless, either. And, even more importantly, I think there are characters that each one would appeal to. I just don't think the appeal is as universal as you are claiming.
-MS
