Actually it isn't bacterias/viruses that are spliced, at least not harmful bacteria but they are used due to their simple form of DNA and DNA replication. It is a bit complicated to explain but prokaryotic cells are used (and bacteria are a form of prokaryotes, not viruses). Eukaryotes (most non-microscopic life forms) have a lot of junk in their DNA, a large part of which does absolutely nothing. Prokaryotes are much simple and don't have this junk so they are used in gene splicing. Using prokaryotes, scientists are able to insert DNA and have that DNA replicated due to the simplicity of their DNA. The only DNA that is spliced into food products is the specific section created, none of the host DNA is spliced in.
GMO has a lot of potential uses and I think it would be interesting to see more research done beyond what Monsanto has done. In the US, we simply need more diversity of crops back but in other countries where food scarcity is a real issue, GMO crops could be useful.
You can't out-exercise poor eating habits.