As for me, it often comes down to a question of who needs who. If a client provides more value than you could possibly anticipate in the foreseeable future and

As for me, it often comes down to a question of who needs who. If a client provides more value than you could possibly anticipate in the foreseeable future and as long as it isn't too depleting(and only you can determine this and only for a limited period), then you should oblige. It's easy to say, say no, but when times are tough you should trust yourself when coming to a conclusion on such an issue since there aren't any unbendable rules after all.