Jeff already has an excellent breakdown as to why any amendments which effectively introduce quotas are counter-productive. Just to add my two cents, any amendment which sanctions artificial influence or disproportionate weight makes OMOV a practical failure. To give any subset within the Liberal ranks a pre-determined, calculated influence over the selection would mean that OMOV is nothing more than a bastardized farce.
You can understand the sentiment, and much of what the YL argue is true, in terms of the good work, a progressive voice, etc. However, that is really irrelevant to the principle within the OMOV concept, and there are plenty of remedies available to offset the perceived erosion of influence. If the YL are the vibrant, mobilizing force that they claim (and I agree), then this system merely requires a retool of sorts, which if successful will maintain the influence. Introducing a quote denotes weakness, that the voice of the YL requires help to be heard. In one sense, that's an insulting sentiment.
OMOV only undermines the voice of Liberal youth if they are incapable of maintaining a large membership, if they lack the organizational skill to attract new members. Under this system, the YL are only handicapped if they currently enjoy manufactured support. If anything, this new system can be empowering, because if the YL is what people claim, it should have no problem maintaining its "say" in any selection. If the Liberal Party can't attract more youth into the fold, then that lack of appeal should be reflected in OMOV, because then the party expression is really an accurate read on the makeup of the grassroots. I see the need for quotas as a defeatist attitude, that completely undermines the spirit of the initiative. If the YL are marginalized under this system, it's "fair" qualities will be nothing more than a testament to inherent weakness. If that fragility is real, then one has to wonder why the wanting subset deserves a uneven voice.
If this amendment to OMOV passes, then the final product resembles nothing of the sort and renders the terminology useless. Why bother, if we essentially create tiers of Liberals, inequalities and distortions?