The bottom line is you made the absurd claim, "I'd think that a regular pipe coupler would be the weak point and the first part to break when used to join pipe clamps."
If you're using enough force to break a pipe coupler when used as a wood clamp, you're using way, way, way too much pressure and you'd end up smashing the wood you're trying to clamp before breaking the coupler.
Instead of just admitting it was ridiculous, you kept arguing and changing the subject and adding new variables to the equation to try to back it up.
I filled the holes with one of those "colored pencil" fill sticks and finished the wood with ... something. (it was years ago). The marks - an imperfect color match, I guess - really aren't conspicuous, but I had decided to try for a (little) higher level of craftsmanship when I made the desk. Thus, no holes, except for the dowels, which I think were a nice decorative touch in addition to their mechanical function.
I don't think that there is any way in the world that a pipe coupler would break when using the pipes as pipe clamps. I have use this method to make short clamps longer during my pipe clamp days and there was never a reason to fear a break.
See my post about the Groz clamp deal at Woodcraft. A good cheap way to augment your existing K-body collection without switching to a completely new style of clamp. I have a bunch of the older K-body clamps, and these are virtually identical.
That said, I agree with what Leon said. I don't own any Cabinet Masters, but I have inspected them closely and I do like how they use a smooth bar design rather than the serrated bars used by all other manufacturers. It is easy to get glue build-up on your bar clamps, and it's a pain in the ass to remove it from those serrated bars. The Cabinet Masters use very smooth chrome plated bars, and it seems like glue build-up would come right off.
I also agree with Leon's assessment of the Jet clamps. I have four, and I don't like 'em. The ONLY thing I like about those clamps is the "trigger" that keeps the jaws from sliding around on the bar unless it's released. This can actually be very handy in those times when you "only" have two hands.
I also have some of newer Bessey K-Body REVO clamps, and these are my favorite (keeping in mind that I don't have any Cabinet Masters to compare them to). As Leon said, they are much smoother than the older K-Bodies, with a larger clamping surface (which can actually be a downside in some circumstances; I have a bunch of lamination forms that would have to be modified to accommodate the larger heads on the REVOs). The rubber (instead of wood) handles are also MUCH easier to grip, and that combined with what appears to be better "gearing" on the leadscrew threads allows for much more clamping force, and much easier release when it comes time to remove the clamping pressure. I really like these clamps, but DAMN are they expensive!
ALSO (again), I have a set of Bessey K-Body REVO Juniors, which are pretty handy for light-duty stuff. The head size is about the same as the old Bessey K-Bodies, but the bar is lighter-duty, and the leadscrew is much smaller so you have less tightening force. They also have rather small wooden (instead of rubber) handles which I don't much like, and for some reason the jaws are designed to be not quite parallel until you apply some considerable clamping force. It's like they recognized that the clamps have some flex due to the light-duty bar, so they built some "acquired" parallelism into the design of the heads. Still, pretty handy little clamps for some things, and not nearly as expensive as their big brothers.
Are you referring to the Groz Parallel Clamp Set? Part of the problem is that Woodcraft is in the US. I live in Canada. Between shipping, exchange and duties, I'd get dinged a significant amount. Still, it's something to consider.
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