I'm also of the Single Master, Set (if large) pool, upgrades don't count mindset.
I find part of the challenge of these events is to find a general group of models that could conceivably do most if not all things. I enjoy the challenge of trying to make an "all comers" list. In addition, let's use the Guild as an example. They have masters who are specifically keyed to being better at defeating models from one entire other faction. I would suggest that a no-limits event would highly favor a guild player who had all the toys. Not all factions have that degree of specialization.
I really think that the M2E system with upgrades goes a long way to offsetting any master-specific weaknesses against strategies: furthermore when you add in the randomly limited schemes. In addition, limiting to a single master means that a given player will be more likely to understand their group well, and give a better game (I know I'm guilty of this myself now and again). Plus, we will probably have better painted crews.
I would very much like to see one faction, one master, 70 stones be the order of the day.