@Xorazne
The various Weed and Browncoat varieties are used in Turf Takeover game mode where they add to the attacking forces. Outside of Gardens and Graveyards or Herbal Assault they are unused.
As far as why they aren't used more often, I often here Players comment that they simply don't have any to spawn. A lot of us who talk here on the forums have high Ranks and hundreds of hours invested but I've noticed more lately that there's been an influx, at least on PS4, or brand new players who are still under Rank 250, sometimes even under Rank 100, and those newer players simply aren't going to have a surplus of consumables the way the more long term players do.