Planes can't carry 20kg per person of extra weight 'for free'. The more weight on board, the more fuel it has to carry / use.
If you aren't paying for baggage separately, then you are paying for it on your ticket, whether you use it or not. I don't really have a problem with baggage fees - it's all part of the cost of travel (like airport transfers - and that can be a MUCH bigger problem in the states, where some airports have very limited public transport options).
But it would make life a lot easier at times if you could pre-book / pay for your baggage at the time you buy the ticket, instead of making you wait until check-in.