Phrase generators for use with testing grammars that don't use a seed
The tools you refer to are probably already deterministic. Tools that accept a seed probably only employ a pseudorandom generator which is keyed from the seed and nothing else. Therefore, given the seed, everything is completely deterministic. If the seeding process is reasonable, then a seed like $0$ or $1$ should work. For definiteness, I suggest you take your favorite "random" tool and use a fixed seed, say $1$. Now it's completely deterministic.

One reason you'd want a deterministic tool is reproducibility. You want to be able to reproduce phrases on demand. If the tool is random, that's impossible. However, if all the "randomness" is encapsulated in a user-entered seed, the output is reproducible; all you have to do is remember the seed used.

I cannot think of any other reason you'd want your tool to be deterministic, but if there is one, please let us know so that we can understand your question better.