it will steal c02, light, and possibly contaminate the younger plants with any bugs/diseases it has. only bad things.
If the plants are fromm an indoor op yhan I would say they wont make your plant any heathier,but they probably wont get sick or anything.If you brought a sick plant in than it would probably contaminate otherplants.If you see a sick plant in your growroom try to isolate from the other plants.Bringing outdoor plants inside however,could lead to devastating consequences.They could bring mold,parasites,mites,and other diseases.