First off are signal dropouts caused by moving from one cell and then into another? and when you move from an open area to say inside a building or behind a hill or a 'dark spot' where there is less signal penetration....
Would allowing you to roam between networks when the signal level drops below a certain point solve the problem and if it would why aren't the networks allowing this to happen? It surely wouldn't be a cost to them to allow for this would it?