It risks creating a system of apparatchiks, power and collusion that ends up favouring friends or the group. The MP becomes accountable to the party. If the MP was designated because he is a list member, his allegiance or accountability will tend to be toward the party.
The parties do extremely important work to develop platforms based on certain values that fracture society between the left and the right. They mobilize people, develop and support dialogue. They train citizens and engage in political socialization. It's their job for civil society. Afterwards, however, they have to separate themselves from the government. They developed the platform, but they must not control the government. That seems dangerous to me.