I'm not sure I understand exactly what the OP is asking.
1. If you are referring to using at a wall plate specifically, then a HDMI coupler into a keystone port introduces issues with the space behind the wall and the depth of the HDMI connector. More than once I've run into HDMI keystone inserts which point the HDMI head towards the back wall, and a heavy gauge, premium HDMI cable simply will not fit into that opening. Instead, I've had to use pigtails to get the wiring in place.
2. HDMI couplers are excellent for pairing two cables, but I generally believe that it is ideal as the poster above said, for stepping down from a long run HDMI cable to a skinny flex HDMI cable for the last yard or so. They really should not be behind walls if you can avoid it.
3. Couplers vs. extensions are an additional failure point for HDMI which can already be tenuous at distance. In all fairness, I've just done a setup with 75' HDMI cables with HDMI couplers, and I will report back what I see when I fire up the BD player into my Denon. If anything is going to sell me on using HD-BaseT instead of a single HDMI cable, this will be the test.
4. They do sell extension cables at some places (M-F) which allow you to not use a coupler, but generally the cable selection is very limited.
At walls, I generally don't use a wall plate with HDMI, but bring the cable through the wall behind devices and straight into the devices. It drops the point of failure down, and if I must use a coupler, it is visible and accessible. I realize this is not as pretty, but it is behind gear so it isn't seen. Also, using a nice bullnose wall plate gives a good final appearance, perhaps even better looking when cables are actually coming out of it.