Why does it crash...
The plugins mentioned - replaygain and coverart-browser are coded in the language called Python.
This uses the gi introspection libraries.
These bug-reports are relevant:
Precise uses an older version of the important package
pygobject - it appears the Rhythmbox developers have used the latest libraries found in Quantal.
I've confirmed that the webupd8 PPA works brilliantly in Quantal - no issues with these plugins.
However, the crash is caused by some-code in Rhythmbox v2.98 that is calling stuff in the older libraries - that call is not handled and causes the crash.
Since parts of Unity and some Gnome apps also similarly use pygobject, the crash appears to leave this package libraries in an undefined state causing the instability.
The RB developers have acknowledged this - but are unlikely to resolve this.
Thus there is a choice - live without these particular plugins and others similarly affected, or to downgrade your Rhythmbox version.
To downgrade from v2.98:
sudo apt-get install ppa-purge
sudo ppa-purge ppa:webupd8team/rhythmbox
sudo apt-get purge rhythmbox gir1.2-rb-3.0 librhythmbox-core5 rhythmbox-mozilla rhythmbox-data rhythmbox-plugin-zeitgeist rhythmbox-plugin-magnatune rhythmbox-plugin-cdrecorder librhythmbox-core6 rhythmbox-plugins
ppa-purge does not clean-up fully and leaves some libraries still at their v2.98 versions.
Finish off by updating your sources:
sudo apt-get update && sudo apt-get upgrade
sudo apt-get install rhythmbox
This will leave you with the default and stable Rhythmbox 2.96 that comes with Precise.
If you still have a hankering for something slightly newer, I have rebuilt WebUpd8's original Rhythmbox v2.97 before that team upgraded.
This is available in my PPA.
sudo add-apt-repository ppa:fossfreedom/rhythmbox
sudo apt-get update && sudo apt-get install rhythmbox