This is worst of both worlds. Compared to naming branches, they both encourage sites to unnecessarily break compatibility, and are held back from developing new features.
They should give 1.9, 2.0, and 2.1 names, and make 2.0 mainline, and call 1.9 "compat" or something and encourage the conditional comments. Then, soon, they should start working on 2.1, which isn't API compatible with a version that's compatible with IE 6/7/8. The current plans for 2.1 are an awfully long time to wait for features that aren't available with IE 6/7/8, for apps and sites that need such things (and have a target audience that is ready for it).
They should give 1.9, 2.0, and 2.1 names, and make 2.0 mainline, and call 1.9 "compat" or something and encourage the conditional comments. Then, soon, they should start working on 2.1, which isn't API compatible with a version that's compatible with IE 6/7/8. The current plans for 2.1 are an awfully long time to wait for features that aren't available with IE 6/7/8, for apps and sites that need such things (and have a target audience that is ready for it).