Agreed , you may be able to dispatch according to the MEL but certain conditions may then prevail that could prevent you from returning e.g single pack operation max cruise altitudes may not be sufficient with prevailing weather, TCAS failure certain ATC units may want to re-route etc etc. As mentioned, a good knowledge of the wider implications of the MEL are required, particularly if engineering is required before the next leg, i.e no fuel indicators or other items requiring sign off before each sectors. Having said that the MEL is guidance before dispatch.