Well, I guess there's nothing wrong with registering it strictly to deprecate it, since folks who use it are not likely to be those who pay attention to registrations, anyway ;)

But text/xsl, as we all know[1] runs afoul of the convention that text/... formats be human readable.  Then again with text/html, the beast has surely escaped the barn.

[2] a sample of such consideration is in  RFC 3023: "

NOTE: Users are in general not used to text containing tags such
      as <price>, and often find such tags quite disorienting or
      annoying.  If one is not sure, the conservative principle would
      suggest using application/* instead of text/* so as not to put

      information in front of users that they will quite likely not
      understand."