The news, reported in the Telegraph, that Nick Brown has claimed tens of thousands of pounds, including £18,000 just for food, in expenses without even submitting receipts makes it clear why we can't trust the political parties to decide which candidates need to go. At the moment, the decision over what an MP needs to do to atone for abusing the expenses system is in the hands of people who have, themselves, been accused of misusing taxpayers' money. The leaders won't want to take serious action, like withdrawing the whip and deselecting those who have made extravagant claims, for fear of having to do the same to trusted lieutenants.
A different, but similar, problem comes up when the decision is in the hands of local party committees. Local parties are often small organisations that have worked with the same MP for years, they will normally be extremely generous in judging whether their MP needs to go. The local association officers in Douglas Hogg's constituency and the local councillors, who are always influential in local parties, in Andrew MacKay's constituency have come to their defence. On this kind of issue, the local party often won't represent the views of the rest of the MPs' constituents.
That's why voters need to be able to recall MPs between elections, and parties need to select their candidates through primaries. Otherwise, voters will often be represented for years by a discredited MP before they are given the chance to express their views at the ballot box. They will then be faced with the choice of whether to vote for a party whose policies they don't support or an MP they disdain.
If the reputation of British politics is going to recover, it needs to become more democratic. MPs can't be shielded from their electorate by the party system.