Except that isn't the entire truth. I'd agree with you if all of the forms had the same stats and damage and the only unique thing about panther was its prowl, but that isn't the case. Panther also has the highest base damage (3d6) basic attack of any non-elemental myrmidon.
Base damage is not the whole story. You also need to factor in the bonus damage that forms get from scaling (At least the level 4 bonus given that Panther requires level 6)
Panther is 3d6 with 1d4, 1d6 and 1d8 bonus meaning it's doing on average 9, 11, 12 and 13 damage.
Owlbear is 2d8 with 1d4, 1d6 and 1d8 bonus meaning it's doing on average 8, 10, 11 and 12 damage.
Wolf/Saber-Tooth is 2d6 wiith 1d6, 1d8 and 1d10 bonus meaning they're doing on average 6, 9, 10 and 11 damage.
Dilophosaurus does 1d10 + 1d8 with 1d6, 1d8 and 1d10 bonus meaning it's doing on average 9, 12, 13 and 14 damage.
Putting Panther only second in terms of damage for non-myrmidon and only 1-2 points higher on average than other forms. Which is not reason to bring the form itself, as it's not like it's a damage machine just because it essentially has a single die up (The same reason why not everyone is running around with greatswords and mauls all the time because of their 2d6 die is larger than any other weapon type besides greataxes 1d12. There's more to damage than simply base dice)
Heck, I'd argue that Wolf would out damage Panther even with its damage defecit, simply because it has Pack Tactics to gain advantage for better attack rolls. While Owlbear can at least match Panther due to getting its 1 damage defecit back from the Str bonus from Enrage.
Panther utilizing Dex instead of Str is only the nail in the coffin for the usefulness of its damage.
This overall means that Panther isn't really a damage machine, outside maybe it's (Bugged) 1d8 bonus from Prowl (Which further highlights its role as a surprise machine, giving you that bonus damage, albeit quite small compared to an Assassin or Gloomstalker)
