Well, the harm is the danger of falling to superstition and fear of omens. If you perform a ritual with no understanding, you're not actually doing anything except giving the words themselves power, rather the carrying out real self improvement as would (possibly) happen were you to do the homework of knowing what you're doing.
Now, I love flowers, people should have them as much as possible. However, if you put a flower on your table just to ward off bad luck (whatever that is)... well, it certainly does no harm to cross to the other side of the street if you see a black cat, or a crack in the pavement, is there? Except that there is. It is paranoid and can become quite harmful, all because someone got advice to put their faith in spells and talismans instead of themselves.
Honestly, I see the blanket argument as an one for ignorance and fear. Bottom line, whatever you do, find out about it. Don't be ignorant and just assume "something" will happen just because of a flower. Want to do magic? Learn it, find out what it is, why it works, if at all and make it your own. This isn't the same as simply trying something until it works. It is a process, it is gradual and it is self respecting.