I hate to see a question with no answers! I suppose you've already done it, but I also did a Google search and came up with very little information, though lots of hits. Still, here's the results. Mainly insecticidal and ornamental, I gather.

Here is a link that might be useful: Tagetes lemonii

Thanks Daisy. I'm happy to keep it as an ornamental and talking point. If you see this plant grab it, very worth while.

Just saw your mention about this lovely tall marigold. I have grown it for a couple of years in a couple of gardens. Its great near a path where it can give off its pleasent scent. Its tough as old boots but It can look leggy in half shade. Give it a regular hair cut to keep a good shape. Now would be a good time to prune it as it will reduce moisture stress on the plant and have lovely flowers for winter.

ps It hates the cold so zone 10 or hotter for best results.

thanks for the tip smithy. I'm not going to prune ours yet as it's flowering beautifully and growing at the moment but on your advice I will give it a haircut the second it starts to look leggy.

The Kings seed catalogue doesn't give any uses for it except as flowering plant. They list Tagetes patula single gold as the best marigold to control nematodes and Tagetes lucida (Mexican mint/sweet mace/texas tarragon)as culinary and medicinal, but Tagetes lemmonii is strictly ornamental.