What you propose is that a user is registered on a site, which basically adds no extra security, unless you do verification, which may take quite the while, to establish with a certain degree of certainty that a user is who he says he is, and then the ease of use is gone, and he might just as well have exchanged cash for btc, or sened money to an exchange.
It's true, the site would need a verification of the person and the ease of use is a bit gone in this way but once registered the user would have an additional channel for the quick trading, and the service provider would have more security sending BTCs, but the paid phone call/short messages provider with the low surcharge would be needed.
It is true that once registered, the user would have a channel for getting easy and fast bitcoins. But if the user at all plans to buy any sizeable amount of bitcoins, he would be better off exchanging locally for cash or sending money to an exchange.
There may be cases where someone have the cell bill covered by their workplace and starts buying coins, but not sure if employeer will be too happy if bill start to get too high!
And there may be other cases where users will dumpt phone credits for whatever reason, and rather have bitcoin (leaving the country, having no use for their prepaid phone service any more), or emptying credits if they are expiring (?).
But frankly, I think sms purchase would mainly be for those just wanting to dabbling into bitcoin, but of course I could be wrong. I guess some people would not care if prices was twice that of an exchange, but I would think most people would find that price too high.
If I remember correctly, it was impulsepay that I implemented the sms service with, and everything worked fine, it's just that the possibility of scamming seems to be too high. And when chargebacks can occur as long as 90 days after the sale, then that doesn't give the operator much room for stability in regards to the finances. You may be churning along quite happily, and then one cell operator sends along a batch of chargebacks, and suddenly you're out a sizeable amount, that's not fun.
It's also the same reasons why all operators selling bitcoins for CC's are very short lived. Here, you just have a middelman, the phone operator as a proxy, so the risks are basically the same. Also, I don't know quite how it works in the UK, but in some countries you can have a phone subscription on credit, and pay the bill after usage, so you could actually rack up quite the large bill the first month, and if the account was fraudulently aquired, there's no recourse for the phone company, and they surely will not pay the payment operator, which again will not pay you, and you're again out of bitcoins.
This will happen, trust me. Once there's a way to get 'free' money, someone will exploit it.