Short answer: The heel brake is great for beginners.
With or without a brake?
Not all skaters use a heel brake system. The reason for not having it is because if you do crossovers it actually gets in the way.
The point where you can take off the heelbrake is when you master the T-Brake stopping technique.
When to replace them
If a brake pad wears down, the angle that your foot needs to be in to brake changes. This is also a preference that an individual skater can have so this part is up to you what angle you like and when you want to replace it.
For sure you need to replace then once you reach the brake holder, when you get in to that section the braking power goes away and you might not be able to replace the holder part.