I think it's builds trust to signal to your users that the product is still young. That sets expectations that there may be bugs but you're looking to actively fix them, and that half baked features will become more stable and usable over time.
The phrase "beta testers" isn't great though. People don't want to pay to test other peoples software. I think "early access" is better phrasing. I'm working on an idea and planning to preview a feature grid with different tiers, then grey it out and have an overlay saying something to the effect of "early access only right now, open your account now and get locked into this lower price." Followed by a list of available features and "coming soon".