It's simply this. Right now the way it works is that this is just quorum for the purpose of getting testimony. In order to be able to receive testimony, you have to have either two government members and one opposition member, or two opposition members and one government member. That opposition member could be any member of the opposition.
Under the amendment, that member could only be from the New Democrats or the Conservatives. That means you could not have the committee sit to hear testimony if both the Conservatives and the NDP didn't want that. But for the sake of argument, it might be the case that the Greens or the Bloc wanted to have testimony and the government thought they were willing to go along with it. Effectively it diminishes....
To some degree, leaving it as is, not putting in the amendment, diminishes the capacity of ourselves and the New Democrats to prevent testimony from being taken. On the theory that in Parliament we should always default towards openness and more debate, I'm simply defaulting towards that direction.