All, not to muddy the waters, but think about where we'd like to be in the future - able to build support for several platforms into one kernel. Device tree is one of the mechanisms to help achieve that as it helps us move away from code laboriously adding the same devices in per platform ways.
OK, so who actually wants the same kernel to run on several platforms? I think that (a) anyone who wants to do testing and (b) anyone interesting in supporting enterprise computing. Frankly, none of the mobile boys care, they are happy doing what they do.
If I put my commercial hat on, I care about ARMv7 and Cortex-A15 platforms. I care about Cortex-A9 platforms as that's what the Linaro members have today. That covers enterprise and networking.
My view would be that we should move towards being able to build support for several platforms into a single kernel. The question becomes 'do we allow non-device tree platforms to be included in a single kernel?'. We could take a hard position and make device tree mandatory or a softer position and not rule out non-DT platforms. The answer to this depends on how clean the end result is and how much working around non-DT platforms has to happen. If it's a lot of work and the results are an ugly compromise, make single kernel device tree only...
Dave