Well, for one, you'd need to know the number of genes the target has in order to hit the last gene, and since you can't aim viruses, you don't really even know what the target is. Gene number one could be even more lethal, since it would insta-kill all SG bots with .delgene inc .delgene inc (they'd delete themselves).
Aside from first, last, and random, I'm not sure if I'd want to target anywhere else.