Well, if internal testing were enough to iron out every bug, the game wouldn't have had errata and a fair amount of clarifications over the past few years.
Even the 0.01% of their fanbase is, quite likely, an order of magnitude or two bigger than their internal playtest team and allows for better results in a shorter amount of time.
The amount of stuff they're changing ( i.e. the addition of weapon mastery, the revamped feats, stuff like the rogue getting "cunning strike" in addition to sneak attack) isn't really insignificant, in terms of balance.
Asking us to only playtest chunks of the rules in isolation doesn't seem like the better course of action, if they're still in the process of revamping the basic framework of the game, because that "bigger picture" you're pointing at is also subject to change.
For example, the revamped feats, which we were presented in one of the first packets, included a modified version of "Great Weapon Master".
All the feedback provided about how the new feat , which was given when we could only evaluate the new version in the context of the 2014 PHB, becomes less and less valid as new versions of the base classes, especially the martials, are released.
Until they provide a cohesive, comprehensive playtest of how the new version of the game is supposed to be played, any feedback we might provide is, at best, inaccurate.