Christ, if my X was a halfway decent parent, I'd be more than happy to share the DC 50/50. I'd practically kill for some more time off.
I wouldn't care if he fed them McDonald's twice a week, let them stay up too late or play computer games for too long (well I would, but that wouldn't stop me letting them go there). I might just have a slight problem with the fact that he's an abuser though. 
Bearing in mind that something like 1/3 of single parents have experienced DV and that only about 1 in 10 victims of DV report it to the police, I'd say the figure of parents denying access for good reasons is much higher than 20% Xenia and that's just for abuse - let alone other good reasons for denying access that have nothing to do with abuse.
Then we have the low level alcohol/substance abusers. Maybe not enough for a full-scale breakdown of their life or to attract agency involvement, but enough for a mother to be legitimately concerned that the child may come to harm.
Then, we have relationship where the mother has done all the childcare (e.g. bathing, dressing, feeding, HCP appointment, school requirements/level/communication, etc) she may have genuine concerns that the father won't know what to do. This doesn't mean the father should be denied the opportunity to learn nor that he's incapable of handling this perfectly well, but if he couldn't do all these things when the parents were together, why would the mother have any faith that he'll take it any more seriously now they are apart?
I do think that some mothers deny access unfairly but with good intentions - i.e. they believe that they are acting in the best interests of the child even though they might not be. For example, if a couple split because the father hasn't changed his lifestyle to accommodate for having a child.
Examples of this might be still going out with the lads every friday night and so missing half the weekend with his child because he's hungover. Or spending his leisure time playing on the computer/playing golf to the detriment of interacting with his child. Or when mum goes out and it's his turn to look after the child, ropes in his own mother's help rather than be solely responsible. I can totally understand why a mother would deny contact under these circumstances. If the behaviour is enough for the woman to refuse to tolerate (thus ending the relationship) why would she consider her child has to tolerate more? I'm not saying she's right BTW, but I understand it, and quite frankly if the irresponsible behaviour of the father is the cause of this, I'd say the onus should be on him to explain why that won't apply to his post-separation parenting. Mediation can help a lot with this of course.