I do agree that both Labs and Testflight have been a little under-used (recently).
Labs should be used for completely new and innovative features that you should (and currently do) Opt-In to experience and provide feedback, that way some tweaks can be made before full roll-out. (@simonb has hinted that the AMEX feature will be coming to Labs)
Testflight on the other hand I think should be used for large-scale and possibly breaking coding changes to the app, where these things will be looked at for the next month at least. Think of it like when a new iOS comes out and you can get the BETA for it - you could literally lose everything but you take that risk. Whereas, things were it is just a case of rolling-out I don’t think should be put through TF as this just exacerbates the time in rolling it out. If its ready and not a major change under the hood of the app, why TF it?
I don’t think people who use TF should be “rewarded” (the complete wrong word but hard to articulate) just for using TF if there is no need to, if that makes any sense?