clearly mark has been successfully using netflix using 3g broadband, so this isn't true. Too successfully.
The issue here is the 25 gig download limit.
Netflix will have different quality settings, High Def, Medium and low, going from 2gigs per hour, .7 gigs per hour to low which might be .3 gigs per hour.
As the wii can't do HD, if you stick with the low setting you should be fine, you could watch 3 hours at the lowest setting for 1 gig, or 75 hours for 25 gigs, or in simple terms 25 movies a month
Which is a lot of tv and movies, obviously you need to leave space for your other internet activities as well. if you stream a lot of videos or download a lot of stuff then this will eat into the limit.
So the answer here is to restrict the amount they use it. Maybe 3 broadband have an area on their website where customers can view their usage for the month? Meteor do. You can check this periodically to see how much the kids have been using the service.
3 also have a 60 gig service, it's 10 quid more than the NBS one.
So why restrict yourself to not having netflix just cause you live in the stix? This is what it's there for.