I find Entourage to be much more powerful than Mail, so every time that I try to switch, I'm left disappointed. That is way I write my scripts for Entourage first. If scripts are simple enough to re-write for Mail, I don't mind doing so. I may do that for the Jott to OmniFocus script, but no promises.
The reason Tex-Edit Plus is a necessary download is because in the current version some of the parsing of the Jott message happens in Tex-Edit. Again, I may re-write the script to by-pass this need, but at the moment you need to download it (again, no promises; after all, the script is free).
I suggest you download Tex-Edit, try it, and if you like it pay the shareware fee. I paid mine a long time ago and have not been disappointed, but that is because I've used it for many scripts. It may be different for you since you don't write scripts, . . . yet! ;)