Time in Primary or "aging" beer

I've gotten my brewing practices down pretty well with ferment temperature control, proper yeast pitching rates and nutrients, oxygenation, etc. so that even my high gravity beers have finished fermenting in less that a week.

In the past I've been giving my lower gravity beers 2 weeks in primary (I don't secondary) and the higher gravity beers 3 weeks.

Is there any point in that, as long as the gravity has stopped changing (and is where I expect it to be)?

I keg, so bottle bombs aren't a concern either way.

Most of my beers are better after 4+ weeks of aging, but I assume that process will still happen even if I keg earlier.

Is there any point to waiting longer in the primary or am I just letting the beer sit around at 68-70 degrees for no reason?

Yea, definitely. With low to medium gravity beers, I typically give 3-5 days after FG for the yeast to clean up after themselves, reabsorb and digest any undesirable compounds created during the fermentation. After that, they're pretty much ready to carb and drink. I make an ordinary bitter I can brew on Monday and drink on Saturday if I really want to. Pale ales are good to go as soon as they're dry hopped, which is 2 weeks after brew day for me (3-4 days of fermentation, 3-4 days of cleanup, week of dry hops).

Certain beers (meaning lower gravity but more complex flavors) require more time for the flavors to really meld and come together (I'm looking at you, chocolate stout). If I'm dry hopping, I'll do that for 5-7 days, again depending mostly on what day of the week it will be when I want to be racking or whatever. Aging is going to happen as long as there is a bit of yeast in the beer (you're not filtering) and the beer is kept warm. Of course you know big beers benefit from aging in that alcohol mellows, flavors meld, and things generally come together better in beers with more intense flavor. This won't happen to chilled beer, at least not nearly as quickly, but you can definitely keg it. A keg is just another vessel that holds beer- yeast can't tell the difference between a bottle and a fermentor (unless you're really super pedantic about it and want to talk about huge industrial cylindroconical fermentors under pressure and all that).