I'd argue it's not my job to "shield the team". It's my job to enable the team to question "dumb shit" and put them in a position where they can discuss these matters constructively with management (or whomever).

Also - I "deal" with underperformers because my team needs to deal with them. It all comes back to "what benefits the team".

I see that there are a lot of different varieties of agile coaches out there.