This is literally the opposite of what I expected looking at the topic as support XP gains are enormous on the abilities that proc them. I do agree that some abilities that don't proc XP should be fixed (looking at you knock-out bomb) I can understand why some don't as you get far more XP from skills that do proc support XP than you do from damage XP. Horn of Valor is undeniably useful, but it would be extremely easy to keep your team buffed for every single kill, granting loads of XP without any sort of meaningful gameplay behind. Yes, you're helping your team, but all you're really doing is a pressing a hot key, every time it goes off cool down without much thought.
Compare this to shieldwall, which grants HUGE amounts of support XP. I guess I've sort "mained" Templar and it is very easy to get top score and definitely top two on the scoreboard after promoting and at level 1 on threatening. Even in a group with level 20 eles and arcane warriors. The difference though, between shieldwall and horn of valor is the meaningful gameplay involved, i.e. balancing guard vs. stamina so you can keep yourself at the front, as well as careful positioning as getting flanked means getting killed.
So on the skills that proc support XP, I think the XP given is sufficient, if not slightly too generous, but I agree that more support skills should proc for XP, and if we're worried about scoreboard balance make it so that more passive gameplay support skills reward less than skills like shieldwall.
In my personal experience, I think its the assassin and alchemist that really don't have their contributions displayed adequately. My assassin build focuses on chaining executions off of Deathblow resets so I get many kills and high killstreaks, but even with 40 or so kills, I rarely score than 3k on threatening, since assassins have no access to support XP, have very little aoe and have to chase enemies around to deal damage, resulting in far fewer enemies tagged and there far less xp (while requiring more effort than the ranged classes).
I typed this on my phone at work so errors were most like made.