The explicit assumption of this branch of science is that DNA is a kind of computing code. Indeed, Canadian researcher Andrew Hessel says that DNA is a “tremendous medium for encoding information: it’s far more robust and compact than even electronic data storage, and it’s really the code of life. So we’re looking at it through the lens of computing [...], which I think is a remarkable shift. [1, 2:17] Seen through this lens, these researchers want to reprogram living organisms to make them do new and useful things. Imagine, for example, “bacteria that breath CO2 and pee straight diesel fuel” [1, 2:58]. These researchers believe that this technology could save the world.
Whether or not you get excited by the possibilities that we may finally live in harmony with nature (by controlling it even more drastically), two things concern me. The first is implicitly outlined in one of Hessel’s comments:
“I think this is the most powerful technology we’ve ever made. The only thing that I think compares to it is electronic computing. And really, we’ve seen how electronic computing has revolutionized and continues to revolutionize the world. I think this is even more powerful because now we’re talking about programming not electronic processors, but living processors.” [1, 3:48]
This technology is the most powerful we’ve ever made… Are we wise enough to foresee all the consequences of reprogrammed organisms? At one level, DNA works like computer code and new (and better programs) can be written, but biological organisms also interact with one another and evolve. Do we seriously think we’re smart enough to understand all these interactions? Taken as a whole, this new field would be orders of magnitudes more complex than the entire Internet, which is by no means simple. This time though, programming “bugs” may be more dangerous than simple computer glitches.
Maybe you think that spreading FUD is not the most compelling line of reasoning. After all, the same has been said about other fields of science before. Nuclear physics was supposed to lead to global planetary destruction, and we’re still here. Fare enough. Maybe we are (or will become) smart enough…
My second concern is not so much about the technology itself but the legal infrastructure surrounding it: we live in a world where companies like Apple Inc. patent things like “rectangle with rounded corners”.  Patents on software are just as ridiculous and detrimental for innovation since:
[They] block individuals from taking part in [...] development and distribution [...] This may not seem relevant to most people but it’s the same as the freedom to write a book. Most people will never write a book, but some people will and society as a whole benefits from what is made by the few [...] 
If the evolution of synthetic biology is inevitable, I hope it doesn’t follow the insane route that commercial software and electronic devices have taken. Exploring such a powerful science requires openness, collaboration, and governmental oversight, not secrecy and commercial control. If we are going to engage in geo-engineering and massive biological reprogramming, the legal model of the Free Software Foundation  is probably the best place to start, if not the only one that will be safe and sustainable.
- Quirks and Quarks: Using DNA to Save the World,
- Wikipedia: Synthetic Biology,
- The Verge: Apple finally gets its patent on a rectangle with rounded corners,
- End Software Patents,
- Patrick Truchon, Free Software,