Shvarz,
you can write any program with the dna language. Mutations don't allow _syntactically_ uncorrect sequences to appear, but allow any possible correct sequence. If the chemical system should work like that, then it should avoid the specification of reactions like:
abc + xyz -> aaa
(let's say that this reaction is syntactically wrong as there are 3 a on the right and only one on the left side, and the other atoms disappeared - therefore the reaction is simply impossible, the expression has no meaning).
What you are saying, instead, is that we should avoid reactions which are _semantically_ wrong, I.e., work but have no evident meaning. That's exactly the opposite of the DB philosophy.
The first and foremost effect of that would be the blurred line between veggies and non-veggies. Bots could evolve to become either. The second effect is that bots can become a lot more diverse: they can be lean and muscular or they can be slow and fat or they can be large and tough (and any variation in between).
See, that's exactly what I mean. Just read your statement, and count the degrees of freedom that this new system will add to DarwinBots:
1) veg <-> non-veg
2) lean <-> fat
3) weak <-> muscular ?
What else? Probably some more, but now, by contrast, try to count the degrees of freedom the DNA gives to robots: impossible!
See (I'm sorry to be sooo boring), that's the difference between selecting syntax or selecting semantics. If you select syntax, you don't know what to expect. If you select semantics, you already know you'll get what you selected.