Symmetric training is encouraged only at higher levels, and only by some coaches. The reason people mostly stick to their base stance in training is because "you need to learn to walk before you can run." In a complex sport like kickboxing, learning to "walk" takes a looong time. Some would say it takes an entire career.
There's no sense in learning a combo from your switch stance if you can't throw it with perfection from your base stance. That doesn't mean you can't try it, but stance changing doesn't do THAT much to confuse your opponent (you asked about being less predictable), so you're better off having a sound offense from one stance, than a half assed offense from both stances.
As you become more proficient you can start having a new combos or moves you like to throw from your switch stance. For example, the step back rear hook (which when you step back with your lead leg, the hook is now your lead hand) is a solid starter switch technique. It leads to a nice cross with your off hand too.
Just because you're standing in your base stance doesn't mean you're not training your lead hand. Practice lead hand heavy techniques! Jab jab lead hook is extremely deceptive, and done entirely with your lead hand. It's a good way to condition your "weak" hand in preparation for switching stances.
All in all, switching stances is a good tool to have in your pocket, but it's not important enough to sacrifice becoming an expert at techniques in your normal stance. Many of the greatest MMA fighters and boxers of all time never switch stance. "Who's going to win, your opponent in their natural stance, or you in your switch stance."
Here's a nice article with some more breakdown: https://www.expertboxing.com/boxing-basics/why-dont-boxers-train-equally-in-both-stances