That makes a lot of sense for OKW. What about Ostheer?
Not too well versed in ostheer, so take this with a grain of salt.
One time to consider it is when you've gone osttruppen. The lmg42 unlock is a large enough incentive in itself to tech battle phase 3. At that point, you might as well construct t4. Since you've already constructed t4, you can then construct the panther if any of the previous stuff I described about the okw panther is desirable.
If you haven't went osttruppen, then theres usually not much of an incentive to go t4. Still, panthers do well against the pershing specifically, which may be a large enough reason to pay for the tech. The pershing (unlike the is2, kv, or churchill; the only other allied heavies) is fast enough to get away from stugs, so the panther, like stated before, gives you the ability to close out a kill on it (which is pretty essential, considering how insane the pershing gets at vet3). Stugs are also pretty vulnerable to the pershing considering they die to two hits + an HVAP shot; because of this, the pershing can usually engage double stugs, take out one of them, then get away. In other words, the panther is probably the best full counter to the pershing and the main reason to get one as ostheer.
Another time to get the ostheer panther is when heavy battle lines have formed. Using stugs when the enemy has AT gun walls is a lot harder to pull off; stugs and panthers have the same range anyway. Things like the brummbar, panzerwerfer, and panther all function significantly better against defensive positions than the t3 options do, giving you a reason to tech up.
tl;dr: there needs to be sufficient incentive to tech up to t4 outside of panthers (unless the enemy has a pershing; then you can tech up solely for the panther), then you consider the stuff in my previous post to decide whether or not to get the panther