I see don'tpanicspoint. For customers who want something that just works and don't want too much hassle if something goes wrong it is worth it to go direct to a network as that way your contract, billing issues and queries go through them and are sorted out by them.
Some people will just prefer to go for the cheapest deal, either because they don't have the money at the time, or because they don't care about customer service and know how to look after their own phone/contract.
I'm the second person here. I will always go for the cheapest deal regardless. And dontpanic is right, these smaller third party companies will usually have some issues. which the actual networks don't have. (e.g stock limitations, refusing to do certain things like trades etc...) which i've all seen happen to people. I know that if a problem comes up with my phone i know exactly what to do and exactly how to check it and get it fixed. Same with the service. So i don't mind going for a cheaper company,