Majoritarian voting gives all the power to the party leaders

A fair system of voting is better for MPs (and not their leader) as they would have more influence, in this system.

Under fptp MPs have no influence, it is like winning the lottery for the party (party leader) which gets elected... they can do almost whatever they want. Having many MPs (under pr) means that any legislation has to meet the approval of a majority in the chamber. The present system is presidential. With pr MPs would be in a better position to constrain parliament, under fptp they cannot reject bad laws because they are (have been) given no power.