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Ok, it might be big. By 'negligible' I meant that, on the first look, I didn't perceive a massive difference between MBA screen and rMBP 13". Not to the extent that I would immediately think that MBA screen is unpleasant to use. Other people have also said that only those who do visual work (i.e., video/photo editing), would benefit the most from retina.

My question is whether upgrading to retina is worth the potential problems it might have with image retention and laginess?

It is simply huge
I use my iPhone 4 along a 10+ yr-old screen from Samsung. It is amazingly good (otherwise it'd be dead by now) but I'm just screaming to get my hands on a Retina. That difference completely pushes me away of the MBA.
 
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