Not familiar with Toki Pona, though it sounds interesting after skimming the Wikipedia page.
I'm currently teaching myself Esperanto. I don't think that just knowing esperanto is ever going to significantly benefit me, and I'll admit that I picked it up on a lark. I like the philosophy of having a universal auxiliary language, though I doubt it will ever get to that point. Mostly I just do it because I'm enjoying the experience, and someday it may be a nice little fun fact about myself.
My previous experiences with learning a second language haven't gone great, but Esperanto is going pretty smoothly. I've probably learned more in about a year and a half playing around in Duolingo than I did from 3 years of high school french class, and I'm kind of hoping I can build on the skills I'm learning here to learn another language somewhere down the line, and I think that may be the real strength of a lot of conlangs, they help you learn how to learn a language.
There's definitely niche roles that they can play, I'm certainly no expert in this area, but I feel like a lot of the autistic community would benefit from being able to communicate in lojban, and people with learning/language/speech difficulties could probably use something like Toki Pona to sort of get the most bang for their buck with a more limited vocabulary. Of course that would require either a lot more adoption of those languages by the general population, or at least a decent network of interpreters/translators to facilitate communication with people who don't speak those languages.