This post is based on the following assumptions as of the present build of Cataclysm beta:
1. To do the heroics you will need most (if not all) ilvl 333 blues, which drop off the final 84 - 85 regular dungeons.
2. Heroics will drop ilvl 346 blues, which will be necessary to begin end-game raid content.
3. Raid content will drop ilvl 359 epics.
4. Rep vendors and/or JP vendors may sell ilvl 333 blues, lvl 85.
Now given this, the only logical reason anyone would be concerned about the JP cap is to stockpile the maximum usable JP in order to buy as many ilvl 333 blues as possible to make the transition into heroics (and therefore raiding) quicker. But it is also logical Blizzard will be aware people will do this, and may attempt to "ensure fairness" between these stockpilers and new players. And the only logical courses to that besides wiping everyone's JP is to inflate the cost of the theoretical ilvl 333 rep vendors so the cap would only yield one or two pieces of gear or require Exalted with each faction to purchase them. If the former, you won't be that far ahead with one or two pieces given how easy grinding will be with RDF (especially when you factor in the daily limit lift doing complete random). And if the latter, then you will be sitting at cap to the point you will lose JP as you grind heroics until you either lose out on JP being earned or be forced to spend your JP on lesser rewards at Friendly, Honored, etc.
So if all of that ends up being true, then what would the point of stockpiling JP to the cap be? I doubt I'll get an honest answer from the lot of you though, at best it'll probably be some retarded assumptive answer designed to maintain your own e-peen by being better at a video game than another human being. At which point logic has little room to stand...