As a Tyranid player, I find this to be ridiculous. I build my army to use Biomancy, and roll on it. Granted, I do get 9 rolls on it across my army. If I don't roll powers I can use, I suck it up and play it the hard way.
Which is indeed what happened in my last game - 1500pts vs Eldar. My Zoanthropes rolled Iron Arms and Warp Speeds, my Flyrant rolled Life Leech and Hemmorage, my Tervigon rolled Life Leech, Warp Speed and Enfeeble. Basically very little to support my army. So I sucked it up, and got stuck in...
And won 6-0, only missing out on 9-0 because I didn't get a troop unit into my opponents deployment zone to grab his objective.
In another game, I hit the jackpot - Iron Arm and Endurance on both my Flyrant and Tervigon, as well as Enfeeble on the Tervigon. Enfeeble and Hemmorage on one Zoanthrope, Enfeeble and Endurance on the other. I ended up nearly drawing that game, despite having an army almost permanently in
FNP and cover (because of my Venomthrope).
This game is fickle. Make your army, and roll on the chart. Be thankful your chart obviously has a beneficial Primaris. Biomancys Primaris is pretty terrible.