It's a tricky one and yes, the definition of excessive or unacceptable is key. There could be good constituency MPs who feel they have no option but to stand down if there is pressure on them to do so. The pressure may, in some cases, be unwarranted - but because of the spotlight that has been shone on expenses, the voters and the constituency party may become over-sensitised to the issue and decide the MP should go. Few MPs would be arrogant or short-sighted enough to cling on in those circumstances. But some constituents have expressed regret at the demise of their MPs for this reason, because they genuinely found them to be good MPs.
I'm not just being an apologist for MPs. I know that some of those constituents will equally have felt let down precisely because they had a good MP, and were upset and disappointed to find they had abused the expenses system. And as I said, there of course some serious abuses of it. And there's no question, some MPs just drift along in safe seats, knowing they'll be voted back in just because it's a traditionally Tory or Labour constituency; some are hardly ever seen locally, or contribute to commons debates, and basically just don't seem to earn their keep. But the electrorate often have a lot to answer for, for keeping those seats safe - there's a lot of lazy voting out there, based on tradition, self-interest, ignorance, or whatever.
But we mustn't forget there are genuinely good, hardworking MPs. Ours is, and some of the people who comment here and elsewhere about how "they're all crooks" will also in fact have good MPs. Knee-jerk negative comments and ignorant cynicism are as unhelpful as lazy safe voting.