This isn't 'top of my head' immigration advice, I am a qualified immigration consultant and I think that you are misunderstanding me.
I am trying to say the following:
1) All applicants need to prove that they can afford to maintain and accommodate themselves when in the UK
2) In order to do this, the ECO's often ask for evidence that accommodation has been arranged, in order that they can see how much the student is paying for accommodation, and can match that up with funds available.
3) I have seen refusals of student visas which say that, because no evidence of accommodation was provided, the ECO is not convinced that the applicant has worked out how much his living expenses will be, and therefore the refusal is based on a doubt that he can afford to accommodate himself. It may not be legitimate, but if all refusals were legitimate, there would never be successful appeals.
I therefore advise nationals from developing countries to provide evidence of accommodation, as it is better to be safe than sorry, and it is better not to give the ECO any possible grounds for refusal. This is especially true when the applicant is staying with family, as is often the case, and will probably not be paying rent.
Vicky