You have to imagine the situation – you are an audit partner and your firm guarantees a $x bank loan – if the bank calls in the loan and the client cannot pay it – it is your firm that will be liable under the guarantee.
Can you be objective/unbiased in giving an audit opinion? Imagine if there is some uncertainty about going concern – might you be tempted to not draw attention to it as you should (MURGC)? – or to not modify your audit opinion if the client does not adequately disclose it in the financial statements?