Are they even worth doing at all? Some of them have helped my execution, but I have been told most of them are impractical for actual matches.
I understand they are trying to teach the game mechanics/engine, but it just seems like a waste of time…
A lot of the trials themselves really aren’t that practical, but as you said, can be good to improve your execution. Whether or not they are a waste of time is up to you really. This is one of those questions you’re better off asking yourself, not the forums; do you have more important things to practice now?
Some of the harder trials are even standard BnB combos, like Viper Trial 24, the later Abel trials, Seth Trial 23, etc
Overall they will help you improve your execution because being able to perform those harder combos (but not needing to be able to) helps you by making some of the more moderate ones seem easier.
With a few exceptions I’ve never really felt that trials should be taken wholesale. I normally just take a small part from each trial and remember that “Okay, this is allowed to work” then use it to help make my own combos later. Like for instance, unless somebody went out of their way to tell me, I probably wouldn’t know that Juri can link j.MP twice in the air. Thankfully the trials taught me that link. It also turns out that is very useful in Juri s combo arsenal
I only do trials for titles 
^Nah, thats what arcade on hardest is for. Get the last one call it a day. :tup: