Tribe pointed out the best post on the issue, written by Mark Mackenzie. Here's a full reprint of Mackenzie's excellent comment:
"Party leaders are currently elected by whom? Party members. Lists will be determined in the same way, by party members. If the no side is to indict MMP on the basis that party lists are provided by party members then I would ask them how they would like to change the leadership determination of their particular party. Incidentally, the ‘power’ of the ‘list MPP’s’ will never approach the power available to the eventual premier (leader of some party) who appoints cabinet, judges, commissioners etc. etc. all without even having been elected directly by the public. Currently, you have three things tied up into one vote - local representative, party leader and party. You may like the local MPP but not the party. You may like the leader but not the party etc. With MMP, you can at least separate the local rep from the party but still do not have the ability to directly vote for a leader. Well, two out of three aint bad but to the ‘no MMP side’. Please don’t attack MMP because the party picks it, unless you are prepared to open up a debate on how party leaders are picked."