Get the feeling Watto's runs are judged based on what the rest of his team were doing around him. Rahul Dravid hit a load of fifties and no centuries at 3 on tour to New Zealand not long ago and received almost universal credit for holding the middle order together, building damaging partnerships, consistently blunting the new-ball attack and making things significantly easier for the batsmen to follow. I mean, you can say what you want about the psychological effect of someone scoring a ton, but when someone scores a series of fifties in a winning cause there's also a pretty large troop of ex-pros ready to exclaim the value of having someone so reliable and consistent at the top of the order to "take the sting out of the attack", "stop the rot" and other such clichés. Even now you have Haydos on TMS saying Watto's doing the most important part of an opener's job by seeing off the new ball. You can't play the "cricketing experience tells us..." card when there's nothing approaching any kind of consensus amongst experienced ex-pros.
It just comes down to the fact that the real value of runs is almost impossible to judge, but also that cricket is a team sport and that value depends hugely on what everyone else in your team does. If Australia were to have two Wattos at the top of the order, that would be absolutely huge for the team, especially against an attack for which swing is the biggest weapon. Ponting would almost never face a ball less than 25 overs old, usually arriving at the crease with the score over 100. A century opening stand to kick off 80% of innings would just give Australia a ridiculous advantage. But with an opening partner who never once makes it past 50 and a woefully out-of-form batting lineup, that same contribution becomes close to useless.
Anyway, I really can't judge at all. Way too many factors and no way of measuring them empirically. If Cribb ever makes a computer program that can mine scorecards for data I'll be able to give you a definitive answer.