Mobile Device Support and Your Application
For the several years (at least since mobile devices were able to browse the net in some form or fashion), companies and organizations have been interested (and increasingly so) in making their sites/web applications mobile-friendly.
But from what little I know, supporting mobile devices is not just as easy as a single differentiation in format.
Mobile devices support HTML, XHTML (various), WML, and other formats to varying degrees. Some tags are supported, some aren't. And that variation basically carved out a niche for companies to attempt to solve those sorts of issues by getting a lot of different mobile devices and then testing sites/content on each. (I used to work for a mobile content aggregator/provider, and that testing and their knowledge of what was supported on each device was the reason that they had relationships with major carriers, media companies, etc. to provide mobile content).
(Relatively) newer mobile devices (iPhones, etc.) are in-general a lot better about supporting HTML than older devices (at least as far as I know), and WML has been on its way out for a good while (as well as the number of devices in the wild that really use it). While there has been a bit of proliferation in mobile content markup languages, as can be seen in the timeline "Evolution of Mobile Web-Related Markup Languages" (from Wikipedia), from what I remember, it wasn't so much about the standards as much as it was how devices supported and/or failed to support those standards.
In general, a "one size fits all" XSLT transform to support most mobile devices doesn't really work that well :) . Applications I've seen that attempt to support a mobile markup language that worked on a single (or very few) mobile devices they tested them against doesn't exactly mean they can place a big "mobile devices supported" stamp on their site, at least not without crossing their fingers behind their backs. :)
That said, should you remove the WML support from your app that you added 3 years ago when it seemed like a good idea? It probably isn't worth the time.
But if there is a question of whether you should improve the quality of your existing services/applications vs. spending a good deal of effort to partially support "mobile devices" in-general (without assistance from a major company that does it all of the time), I'd choose the former, not the latter. And yes, I'm perfectly aware of the many, many mobiles devices out there and the huge market that entails.
My guess is the way that things are headed in the future (granted- some years down the road) will be less about the (generic) web content format provided from the server-side and more about how a site's content should be rendered on a device on the device-side as bandwidth becomes less of an issue. I'd also bet that there will be some major change in format of all web content over the next 10 years to support that kind of thinking. But I could definitely be wrong on that.
As always, I welcome all to provide feedback, even if to say that I'm off my rocker, as I often am. ;)