Doing this could give the plant special new abilities like resisting disease. This is rather like grabbing a large scorpion by its claws because you know about them and what they're for. There may be a 'sting in the tail' which comes from splicing strange genes into other organisms - from viruses to plants, for example. No-one can be certain what will happen. It is unpredictable .
Genes can do unexpected and unintended things and nobody can ever be quite sure what.
Some plants could be made to grow in salty water or very little water - good for very dry countries. Others could be programmed to resist disease . Some could even make stuff called vaccines which could protect kids against nasty illnesses like polio or measles.