When I play an aggressive strategy, I feel my opponent loses a ton of tempo by playing mana crystal/flower. I actively welcome them doing so.
When I play a creature-based attrition strategy, I can usually trade ~2-3 actions of a ~10 mana creature to destroy one of them. It always feels like I'm getting ahead when I do this, and I usually feel free to do this because my opponent loses the "creature war" because I can outsummon them in the initial clash because they sunk so much mana into crystals/flowers.
In my mind the perfect amount of mana generation is +1 over your opponent. When it comes to Crystals/Flowers, I'd almost always rather pay a little extra to upgrade to a Battleforge, Spawnpoint, or Familiar depending on my strategy. So, no, I pretty much never play Crystals/Flowers.