it's getting more and more frustrating that people recommend using a rubbish cube, or even worse BUYING a rubbish cube.
Here are some reasons for getting a good cube:
1. A cube that is easier to turn will facilitate fingertricks. This helps steer the cuber away from developing bad turning habits (wristing and such like).
2. A poor cube, that frequently locks up, can be frustrating to use, so solving will be less fun.. and if it's not fun, then you're more likely to give up.
3. The easier a cube is to turn the less likely you are to develop any medical conditions from it (RSI, Carpel Tunnel, etc.). Just to hammer this point home: THERE ARE MEDICAL REASONS FOR NOT USING A POOR CUBE! (this is coming from someone who took a 7 Year break from cubing due to damage partially caused by a cheap cube).
4. If you're going to move to a good cube eventually, why wait. Good cubes are barely any different in price from rubbish cubes, also the sooner you start using a good cube, the sooner you'll be comfortable using it (any comments about them being 'too fast' are rubbish).
Just because you 'can' be fast with a rubbish cube, doesn't mean you 'should' use one.