It depends on the political system. In many a parliamentary system, voting differently than what you are told means you will not be in the party list ever again, so for the day to day, there's very little convincing effort when you have the majority. And especially in systems where there is very little legal money in politics, the carrot is just ability to get access for not-so-legal money. So party leadership is a matter of having enough businessmen friends, and having will to power in a rather unclean word.

The whole "you are just one vote" doesn't really apply there. As the 30th legislator on your party list, you show up and vote as you are told. And in systems like the one in the US, barring rare moments, it's also not all that different. The card that your party will abandon you, and that if you want a position of any importance you have to play ball means many a junior congressperson just votes as they are told, and spends time fundraising.