This is something that I see frequently come up and nobody really seems to have a definite answer for this.
Can a plant creature with immunity to polymorph effects be a druid and use wild shape?
Can an undead barbarian with immunity to mind-afecting effects benefit from the morale bonuses from rage?
There might be more, but morale and polymorph seem to be the most common cases. Does anyone have a clear answer to that?
No, if you're immune, you can't benefit from things you are immune to. You can lower Spell Resistance as a standard action, but you can't lower immunities.