Really, as long as you have mobile data, there's no difference between supporting SMS encryption and not - you can only talk encrypted to other Signal users anyway, and other Signal users will get your non-SMS-routed encrypted message just fine.
The use case for Signal-encrypted-SMS is continuing to send encrypted while you have no IP connectivity, but thankfully that is becoming a rarer corner-case. The last time I had SMS but no IP was on a cruise ship.
> I expect it to have high battery consumption and an unreliable user experience, but would be fine with it if it comes with a warning and only runs in the absence of play services
You don't get high battery consumption, or unreliable user experience, or a requirement for Play Services with pure SMS encryption.
SMS encryption is actually quite a tricksy problem. TextSecure's entire purpose (over bog standard IM OTR) was to solve it, providing individual message-level forward secrecy. This is why it's so perplexing to me that they dropped it (I understand that part of the reason is that iOS simply does not allow it)