IMO, it's all about the size of the exit pupil. The human eye can typically dilate to a maximum of about 6mm in low light. Maybe not even that far for aging eyes.
You can easily determine the exit pupil of your scope by dividing the magnification into the objective diameter. For example: a 3x9x42mm scope would have an exit pupil of 14 when set @ 3x (which is way more light than is usuable by the eye), but would still have an exit pupil of 4.67 if set @ 9x. If you set the scope @ 7x you would have the magic 6mm exit pupil which is normally the most that you can use even in low light.
Now - if you had a 3x9x50 scope, you would have an exit pupil of 16.7 @ 3x and 5.55 @9x.
So what does the larger objective lens really get you? It gives you the ability to use slightly more magnification while still getting a 6mm exit pupil for use in low light. With the 50mm scope you could have your scope set at 8.3x, instead of 7x (with the 42mm), and you would still get a 6mm exit pupil. In normal daylight it is of little value.
So is a 50mm or 56mm worth the extra weight, scope height and cost - only you can decide that, but the more I understand about optics the less I am attracted to the big objective scopes. Normally when the light gets low my scope isn't set to its maximum magnification anyway.
This may be an over-simplification of the whole principal but I hope you will understand my point. For what it's worth, I've got 40's, 42's, 50's and a 56mm scope.