A certain balance? I doubt that seriously. There's plenty of zone without herbs at all and some are so overabundant that it becomes ridiculous and we toss entire BAGS of red berries or crimson lichen.
How it is so unlikely that crypt herbs would grow elsewhere where there are the same required temperature, soil etc. I doubt a few more mushrooms to replace useless ones would UNBALANCE the system.
That's the balance. Have you ever played a game like the McDonalds Monopoly game? They give you almost every piece on the board that you need to win, but hold out on certain ones. The same thing goes for these herbs; you have more crimson lichen than you could ever possibly want, but those damned bloodberries only spawn one month in the game year, and that's what keeps it balanced (or in the Monopoly game, keeps everyone from winning). You still get to use the crimson lichen in the other recipe that requires it, but it limits the amount of freedom potions you can spam.
You can only make cure moderates most of the time, except during winter. The big healing potions only come during spring, which keeps people from cranking them out forever and ever.
Just because you pick every herb that shows up doesn't mean it's supposed to be useful to you. I've noted the common herbs that spawn all year, and I only ever pick them or buy them if I have the rarer ones that will go with it. I also managed to buy about 30 viccar's caps from people because I didn't want to storm through their low level crypts all the time, so I know you can buy them. Of course, I did pay them a lot, but that's economics, and life.
Maybe you should put up posters saying you want viccar's caps, even when you don't need them, so that you'll always have a supply. It's hardest to get things when you want them immediately... especially in the NWN herb world.