@syvarris: Your analogy with the gun designs doesn't really work because of a crucial detail - genetic modification is fundamentally different from basic weapons design because genetic modification is based on introducing externally-devised changes to a highly complex, not fully understood (not by the player, anyways) system and hoping that they work out in your favor - even when they should, there is always a considerable chance that they will not, and that they will not in highly unpredictable ways. See example of nonfunctioning protein robbing a person of thumbs (but not fingers) and causing several other abnormalities. Then there's variable gene expression, quirks with the proteins in question, all manner of hideously complex issues that could arise when dealing with a system as complex and based on coordinated functioning of virtually all tissues as a human being, that kind of thing. There has to be a random element in genemodding, especially when treading new ground, since there is no realistic way to come up with logical objections to a particular design. And thus, where no knowledge can be had, randomness must take hold. The roll could only be made to determine initial problems with genetic designs, and then corrections can be performed, though if those involve additional modding, one could roll for those as well.
There's just so many unknown factors (to the designer - the VR, if one allows genemods to be designed in it, will have fully modeled the resulting DNA topology, predicted resulting expression, epigenetic effects, likely folding of proteins and similar things, although a fun variation would be that VR does not do this, at least not perfectly, and there may be unforeseen side effects if you use it to create a genemod) involved that they may as well be random. Otherwise genemodding becomes a do-anything method, only limited by the upper limits of nanotechnology, which are, to be frank, rather lofty when properly applied. And if you want your genemodding to do virtually anything, why not just walk up to the Doctor and ask for a special treatment to be performed with his space magic?
Although I do concede the point about that whole fallibility of the GM thing, hence why the rolling (for genemods and nothing else, I will stress), in my opinion, becomes more necessary. Sometimes you'll luck out and things will work as intended without much issue (probably more likely with simpler mods), and sometimes that spiffy feature suite you encoded in your DNA will just plain kill you horribly or inflict spongiform encephalopathy upon you when you try to test it in real conditions.
Should note, however, that just because something already happens and will continue to happen, doesn't mean it's ultimately a good thing, however. I'm quite the proponent of the idea that if a GM doesn't know, then the clear answer is to roll for it to attain certainty. Possibly without revealing it to the player in question. Might make field testing and prototyping more relevant if applied to all esoterically physical inventions as well, anyway.