What exactly is his character meant to do?
If you just want a bard with lots of flexibility, I think the Polymath muse got you covered. In fact, between the muses and the ability to pick up all three of them via feats, I think the bard is easily the most feat-starved of the casters. I have little trouble fitting in at least one multiclass into any other caster. But for the bard I can hardly get all the feats I want.
So unless you have some specific thing in mind that simply doesn't work without rogue multiclass, I'd stay pure.