I think one of the things to examine is why you might be using a particular power over other choices and what that means given those weapons given those classes.Arkhoisan Fang student can be used with the Great Sword. Which is one of only 2 +3 proficency 2 handed military weapons. The other one being the Falchion, which has high crit, but does in average 0.5 damage less per W. (And on crits 2 damage less per W but +5 per tier. So depending on the weapon attack you do it might still deal less damage on a crit. (Like a level 9 3W attack deals still less damage on a crit.) And Bastard sword is together with Triple-headed lail the only +3 1d10 (highest damage) one handed weapon in the game and the only heavy blade at that. So if you want to play heavy blade 1 handed this is the best choice. (Sure some optimization guide might say "uh flail are better", but not everyone wants to use flail, and the flail feats have different properties than the heavy blade feat.
First off, two-handed +3 proficiency military weapons are generally bad. So is Bastard Sword for Rangers, especially if you're building yourself as a charger. You're giving up a hand or a feat, at least get reach for your trouble.
Warlord & Ranger — the reason they're using those powers is that they're out of position for their primary choices for some reason. Even if they have available bloodied targets, they're probably not actually using those powers anyway. Warlord should either be granting attacks or bonuses to damage/hit to their striker friends to finish the bloodied targets off. The Ranger should be doing Twin Strike.
Fighter with Footwork Lure is using an Alhulak, because they can prone targets with it with Flail Expertise. Or a polearm/spear for Polearm Momentum.
In every case, this is not particularly ideal use of a power.
And that's before using weapons that are bad. Use a longsword, glaive, fullblade or drow long blade and 90% of the time, one of those 4 choices is better than any of the 3 heavy blades listed in this feat. And that's before considering non-heavy blades such as Rapier+Swift Blade Style for Str/Dex Rangers, reach weapons for Warlords for Commander's Strike or the already mentioned Alhulak for Fighter doing Footwork Lure.
So you're a Warlord getting opportunity attacks for unexpected reasons. Because they can just punch you which is usually an extremely good move. So after spending 2 feats and taking generally bad powers, you have a corner case for where this is useful. That's Purple.And for the warlord being able to use encounter attacks as opportunity attacks means that they can use more at wills (like the powered up one in the feat before), which can be quite useful. And letting allies move (as 2 (and a half) of the encounter powers mentioned do) can help you protect your allies better, because now an enemy running past you has a really hard time to reach your friends behind.