Adam, are you basing these numbers on the WAP detection by the varnish? (Based on the "accepted" http header not containing "HTML" keyword), or is it based on the user agent detection?
Right, the WML support detection in Varnish and MobileFrontend consider something WML only if "text/vnd.wap.wml" is in the Accept header (WML cannot be served if it isn't present) yet "text/html" (which also implies HTML support) is not at all present.
I see Max provided further inferences and his read on opportunity costs in another reply on this thread. There are some other devices that get used in the course of routine maintenance of MobileFrontend outside of Android 2.X+ and iOS (e.g., Nokia/Symbian/BlackBerry/Firefox OS class OSes). Incorporating your feedback and Max's feedback, I wonder if obtaining a few additional device, specifically in the cheap + popular + claims both WML&HTML support trifecta, just to validate the basic browse/search functionality on HTML periodically would be an efficient use of time.