Probably the biggest advantage of sim-only is you can keep your current phone. This is especially useful if you have an expensive handset. In the greater scheme of things they haven't changed too much over the years, functionality is much the same as it always was. And with an end to subsidised phone prices we all know how expensive a new phone can be. So why enter into a new phone contract if you already have a perfectly capable one?
With a sim only contract you're not paying for something you don't need, and of course you are free to change it further down the line without it affecting your contract in any way. If, like me you prefer to buy your own phone outright and not be tied to what your chosen network has to offer a sim-only contract again makes good sense and offers excellent value.
Don't forget we sell a wide range of mobile phones, most of which are unlocked so will work with any sim card, any network, contract or pay as you go. Compliment your sim contract with a new phone when you're ready to buy, not when you need a new contract.