I would not consider mosses to be a weed. They might grow rather quickly compared to some other aquarium plants, but this growth can still be controlled and they do make great sinks/sponges for excess nutrients in the water, plus shrimp love to look for food in them.
The fragmentation isn't nearly as bad as your friend implied. While some parts may fall off if the plant isn't properly secured, he might have been referring to the fragments ability to survive and regrow. Unlike a good percentage of aquatic plants, mosses don't need a root system to survive as they take their nutrients directly out of the water.