Pummel is not what I thought it was.
At the outskirts of Camp Bloodshore, I punched a blue con Ladybug once with Heavy Strike and once with Pummel. Heavy returned 761TP, Pummel over 1200TP. Clearly, then, Pummel is a TP hoarding move--but! Then I went to punch a blue con aldgoat nanny. Heavy returned 785TP, Pummel 493TP. This is with no misses and no extra TP from hits, evasions or TP enhancing abilities.
So, what gives? I never noticed until now, but PGL does not always attack twice. A single Heavy/Light/Pummel input does not grant the "Number of attacks: 2" on our weapons all the time. The number of attacks that connect will award TP accordingly; a 2-fold Heavy Strike probably will outdo a 1-fold Pummel in TP gain on the same target.
Things get more complicated: I next engaged a yellow con crab and luckily got 2-fold attacks from both Heavy Strike and Pummel: Heavy returned 333TP, Pummel 392TP. Pummel offers no advantage against this target at all.
I have good reason to believe the promised TP bonus from Pummel varies with other TP gain mechanics. Superficially, we can easily see how crabs return notoriously little TP, whereas "softer" mobs return more. This plays to Pummel's benefit. In the case of the ladybug, Pummel awarded almost 100% more TP than Heavy Strike for only 33% more stamina--talk about a bullet train to Victimizeville!
Since both Heavy and Pummel deal the same amount of damage and have (presumably) the same chances to punch once or twice, I will conclude by saying Pummel is no replacement for Heavy Strike like some kind of "Heavy Strike II" for the sophisticated PGL. Pummel is not useless, though; it costs no AP to equip so its effectiveness can be gauged on a new mob type in a round or two.
As usual, sh*t is situational.