I don't like banning things outright, unless it's for a punishment. That's because my 10 year old son goes through so may crazes - one week he's into his Xbox, another week he's into his Gameboy, another week it's dragonball Zee on television. Right now he has no interest in playing on his Xbox or Gameboy, despite it being the holidays.
I think it's important for him to feel he can go with the flow. I don't want him to have too many rigid rules if I can help it. Obviously if homework and exercise are not happening and his behaviour is not good, then the offending item is banned or restricted.
At school, he has to follow strict behaviour rules, all day he has to follow a timetable. It is all laid out for him. He has to work hard. As well as that he has homework, an hours' extra coaching on a saturday for the 11+, 11+ homework, cubs, youthclub, church, after school football etc. Not everything every week, but lots of his time is ordered already. I strongly think he needs to have freedom each day to do just as he pleases. Or nothing at all if that's what he wants. If I did not have some time off each day it would do my head in.
Also seemingly trashy pursuits can lead to unexpected creativity: my son's interest in yughio, pokemon, dragonball Z cartoons has sparked off and interest in manga drawing. He has spent hours and hours with pencil, paper and a 'how to draw dragonball Z characters' book, and then that lead to him wanting do more life drawing.
I haven't mentioned my 4 year old - much of the above applies to him - I feel he needs time to switch off in the evenings, and if he chooses to do this in front of TV, within reasonable limits, that's ok by me. Also, both boys often do other things while watching TV - drawing, lego etc.
I have known some children who far more consistently into Xboxes or videos etc than my oldest son, so it's hard for me make general statements about restricting TV/computer viewing. Faced with a child who was not happy doing anything else, I think I would consider a blanket ban on them.
I also agree with philly that it's wrong for year 5 children to have homework that can only be done on computer. It's not fair on those children who don't have access to a computer at home. They are too young to go alone to online centres and libraries to use a computer.They are dependent on their parents' support in taking them to a pc. And some parents will not/ cannot do that.