Are there any advantage to having a high galactic readiness rating in multiplayer?

Galactic Readiness has no effect on Multiplayer.

That doesn't quite cover it though; there is no Galactic Readiness in multiplayer (you earn it, but it doesn't change anything). Galactic Readiness exists solely within Single Player.

But, if you've maxed Glactic Readiness, you get an XP bonus at the end of a match instead of a readiness boost.

If you are interested in playing multiplayer it is better to keep your readiness at 100%. Because in doing so you get an extra 5% experience from every match played. But other than the bonus experience you get, it is only useful for solo play.