The very definition of hinder is to create difficulties for (someone or something), resulting in delay or obstruction.
I don't know how that can be any clearer for people. The issue I have personally with healers who "only" choose to heal, or ANY role who "only" chooses to do the minimum, is that they're being selfish/lazy and hindering my playing time. I don't have nearly as much time to game like most people do, so when I do I want to progress and get things done efficiently to keep the workflow going smoothly. As a boss you wouldn't hire an employee if you knew they were just doing enough to get by would you? No, you wouldnt. Same concept applies here, and that's where the problem derives from. Selfish and lazy players costing others. No one is expecting perfection or pro skills, just the courtesy to play the role they chose FULLY.