[linguistics__]

To answer that there are two main factors: 1) For this kind of single instance gene transfer to have an evolutionary effect, it will need to happens to germ-line cell (so an egg or sperm cell in a multi-cellular organism, or in a single-celled organism). Otherwise the mutation won't be inheritable and it won't be evolution, just mutation.

2) The organism(s) affected by this gene transfer would need to be under some evolutionary pressure that would benefit from extra genetic variance. Genetic variance in itself is not a driver of evolution, evolutionary pressure is, which does need the variance to actually do anything. Pressure is the sculptor and variance is the clay, without one of those there won't be a statue.

Also, normally most random mutations are either negative or neutral (there's a lot more ways an organism can be dead than alive), so I would suspect this might also be true of gene transfers. Though gene transfers from environmental DNA isn't as random as direct genetic mutation, so perhaps there's something interesting happening there. But I wouldn't be surprised to find out that the risk of cancer or viral infections would be higher.