[NTG-pdftex] \mathsurround

I recently needed to typeset a narrow column with some math, so very dificult to find break points resulting in acceptable looking lines. I found that the best solution would be achieved if I had, say, \mathsurround=0pt plus2pt, but \muthsurround is a dimen, not a skip. Is there any particular reason why it is not a glue? If there is not, it would be easy to implement, wouldn't it? --Javier