What I'm talking about are the JP we earn at max level. What are they for?
There's one thing they're clearly not for: getting into heroics. Suppose you get to max level with only a nominal amount of JP (you're a new character leveled mostly by questing). You can't hit the 440 item level threshold (you lost some of your quest gear, let's say) and can't afford the AH prices for crafted gear. You'd like to replace a slot or two with JP gear.
The only way to earn JP at max level, if you can't get into heroics, is scenarios: 30 JP per scenario. At roughly 15 minutes per scenario, that's 2 JP/minute. The cheapest JP items are 1250 JP, so it would take you 625 minutes, or 10 and a half hours, of just straight grinding scenarios just to buy one JP item. That's not counting any queue times. Not only is that completely unrealistic, it's probably the worst conceivable time investment in the game. You could buy 4 or 5 equivalent Honor items in that time. Obviously, JP cannot possibly be designed to get new 90s into heroics...or if it is, it's designed to be drastically inferior to Honor for that purpose.
So, assume you can get into heroics. Then the primary source of JP is running heroics. That's actually not that terrible a source, especially considering you're also getting chances at gear drops. But once you're at that point, what are they for?
They're not for making heroics easier. The only way the heroics could be easier (excluding of course the trash at the end of SPM) is if the bosses just killed themselves when you walked in the room. You don't need JP gear to run heroics. So what do you need it for?
Prior to this hotfix, JP could have had a purpose: to be converted to Honor. But now both the Honor and the JP gear are sitting 5 item levels below the minimum gear requirement for the first LFR instance. If you want to raid LFR with slots filled in with JP gear, you also need to get raid or valor gear to compensate. But if your gear isn't good enough to run LFR, why is it good enough to run normal raids? And if you need valor gear first before JP gear is useful, doesn't that entirely defeat the purpose of JP being the easy/accessible 'farmable' currency?
This whole system makes no sense.