Despite your feelings on the issue, the difference is quite stark. Relying on allocations in yearly budgets, subject to the political whims of lawmakers, is clearly a different animal altogether.
Even right now, the politicians can choose to enact legislation to modify or remove the tv license fee.
It would have the same effect. For the duration of the charter, BBC would be guaranteed funding without interference from the government. Only difference is method of collection and in my view, reduced cost of collection.
If you are an agency which wishes to retain some modicum of control over your own destiny, then you most likely wish to retain some modicum of control over your sources of revenue.