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Unless a +1 encourages viewers to become complacent with their viewing habits, and instead of 10% more viewers you merely end up with 10% of ITV1 viewers delaying their viewing while they do something else, because they can.
If a +1 channel makes enough revenue through advertising to offset it's carriage and additional rights costs and in addition make a little more profit, it's certainly much more of an economic proposition than an original content channel would be.
Unless a +1 encourages viewers to become complacent with their viewing habits, and instead of 10% more viewers you merely end up with 10% of ITV1 viewers delaying their viewing while they do something else, because they can.