Originally Posted by Stuart

Which would cause massive problems for commercial TV, possibly bankrupting several channels (including ITV from what I have heard from people in the industry).

Simple economics.

I mention economics because when you introduce more supply into a market with a fixed size, prices go down. Advertising budgets are finite. They will not suddenly increase because the BBC is accepting advertising.

In short, you would suddenly introduce a far greater supply of advertising space to a market that is already saturated (thanks to hundreds of digital TV and radio channels), and the price (which those companies need to stay relatively high to remain profitable) will crash.