The relationship between
polyglossia with respect to XeTeX is complicated. The general rule of thumb is that if the babel
.ldf file uses non-Latin scripts, then you should use
polyglossia and generally can't use
babel but if it assumes Latin scripts, you may still be able to use
babel. With respect to your specific question, about Russian, it's obviously possible to write
.ldf files that work with both engines, but for most of the non Latin scripts, this will not have been done and
polyglossia will still be required.
The issues with Cyrillic are arguably less complicated than those using RTL scripts such as Hebrew and Arabic (as alluded to in @egreg's comment). Other scripts such as for Hindi also pose special problems for which pdfLaTeX is unlikely to be a choice. So
polyglossia will remain required for those scripts which independently depend on XeLaTeX.
In 2013, Javier Bezos released a new version
babel which (among other things) provides better support for UTF-8 engines such as XeLaTeX and LuaLaTeX. This means that the differences between
polyglossia for most Latin script based languages will be minimal, but for RTL languages and non-Latin script languages generally,
polyglossia support may be a better choice. The current version of
babel (3.9 at time of writing) provides "minimal support for XeTeX and LuaTeX".