If you find multiple people acting like that, you can loosen your hand requirements because they're not "charging you" for draws.

If it's really prevalent, that's good. You can play offsuit connectors more, even the ones with gaps in 'em (86o, that sort of thing). Add Kxs, even Qxs from late.

it's still very difficult to really judge what someone could have if they're being deceptive like this
Yes, that's the advantage to passive play. The DISadvantage (it's a biggie) is that it doesn't matter all that much what they've got, since they're constantly giving you free chances to make the nuts at 'em.