Bacteria with fully synthetic genomes are unlikely to pose a threat to us.
The big advantages in creating these organisms are that you get a cell with just enough genetic information to perform basic metabolism and reproduction and the entire genome can be completely understood. From this basic template, new functions can be added to create a well-behaved and well-characterised cell which churns out human insulin or growth hormone or whatever and nothing else.
Wild bacteria can be modified to produce these substances, but the genomes of wild bacteria have been evolving for over 3.8 billion years and can have all kinds of quirks and tricks hidden in them. A wild bacterium that's been modified to produce human insulin might suddenly switch from swimming freely inside a reaction vessel to forming a biofilm on it's inner surface or packaging itself into an endospore when the PH and temperature drift beyond a certain range, or it might start producing a lethal neurotoxin when a mutation reactivates a pseudogene that's been lurking in it's junk DNA. With a synthetic strain, it can be ensured that if the conditions drift beyond certain tolerances or if a cosmic ray zaps a particular stretch of DNA, the organism will simply die and won't pull a billion-year-old trick out of it's ass.
If a synthetic strain escaped into the wild, it would most likely die immediately from simple environmental exposure, and if the environment didn't kill it, it would be hopelessly outclassed by native organisms. It would be like creating a blind-deaf-dumb-quadriplegic-retarded-albino human, raising them in a bubble and then dumping them in an unpopulated savannah in sub-Saharan Africa. Synthetic strains would also be crap starting points for bioweapons; it's much easier to take a wild bacterium and make it more deadly than to take a synthetic one and make it even slightly dangerous. Before you even started to add exotoxins, antibiotic resistance and immune-system countermeasures to the organism, you'd have to add basic features such as the ability to survive exposure to oxygen and UV rays, which would be lacking in a cell with a bare-bones synthetic genome.
