Bob, just to be clear, are you objecting to the joint on the first leg, or do you think all of the legs I posted are unacceptable?
First off, thank you for taking my critique as intended. Im sincerely NOT trying to be rude or beat you up or anything silly like that.
In the picture, the one on the far right side seems perfectly fine (from that face at least). The two middle ones are "ok" the one on the left I wouldn't use.
I apologize for being pedantic here, but this is whats going on. Structurally, PVA glue forms a bond with itself, not the wood. The glue will penetrate the wood fibers and bond to itself, which is where its strength comes from. A proper glue joint will rip apart the wood before the glue fails. When you have gaps that large, the glue is unable to polymerize with itself properly and creates an extremely weak structure. Furthermore, PVA glue is EXTREMELY susceptible to moisture as well as temperature gradients, creating another failure point. Even titebond 3 will fail under moisture given enough of it and enough time. Over time, those gaps will develop into large cracks, and it will eventually just fall apart.
So whats going on? I have a few theories.
1) Despite being a "hardwood" poplar is incredibly soft, and is inherently prone to warping. Sucks to hear it, but you MUST let your wood acclimate to your shop before chopping it up. What happens is the wood at the store will have a different humidity and temperature than what your workshop has. When you then cut it, it'll expand or contract, causing bowing and cupping, which is why its so difficult to get wood straight and keep it that way right off the bat. Also a lot of woods have internal tensions, and when you cut them, they will dramatically expand or contract. This is more an issue with figured woods though.
2) Im only seeing a couple clamps here... adequate clamping pressure is also required (see my post above about buying a bunch of harbor freight f clamps). Since it IS a workbench, what I would do is cut the legs about 2 inches longer than they need to be, and screw the ends down. This gives 2 advantages. 1) The boards wont "slip" under glue up, and 2) gives you a mechanical advantage for clamping the rest of the board. When the glue is cured, then simply cut off the ends to desired length.
3) Your hand planing technique is likely off. Flat =/= parallel. See my attached picture to see what I mean.
Your improvement over the few boards youve done have been enormous though, I admittedly didnt see the updated pictures from the first couple, so I apologize for missing that part. I am more encouraging you to develop skills NOW, as it will only help you on future endeavors. While I may seem abrasive, I do wish someone was a little "harder" on me when I first started, as I had to struggle with a lot of growing pains when I first started out. The big thing is the game changes when you start using hardwoods that cost you 8$ a boardfoot and screw them up because you didnt learn techniques properly. You seem to be super receptive and eager to learn, and Im sure your bench will be a great start to your new hobby.