afterwards, someone comes up to you and says something like "great gig, really liked it. You need a different drummer though, I didn't like what he was doing. Hey, I play drums, I could do it!" Or if you're a solo act: "That was awesome, but what you really need is a bass player..."

What's the point in this? If it sounded that great, then the offending member clearly doesn't need changing, and the band is probably hapy how it is.

Why do people try and offer their services?