There are couple of reasons that appeal to me:
Since I have the room to expand the garden, I like to be able to freely walk around and between the garden. But, if the space is limited, and one is trying to utilize every inch of available space, can definitely agree, 4' beds can support a good variety of veggies at the same time.
Wheel barrel a must for me to get between the beds
Two rows of tomatoes in one 4' bed - I find a bit too crowded in terms of the lack of the air flow
Tried this year a 2'x10'x12" bed, with 3 bell peppers plants on each side, two indeterminate tomato plants in the middle, and 4 rows of 6 varieties of Basil n front of tomatoes - results are just as good as I got from 4'x10'x24" bed, same soil, same care, but less than 1/2 of the soil what I'd use in the larger bed.
So in my mind, since the crop rotation is highly recommended, I can build more than double the number of the raised beds of 2' wide with the same amount of soil as I'd use on 4' beds, and easily utilize the crop rotation.
Support of tall, indeterminate varieties I find to be much easier to achieve when planted all in one row of 2' beds.
Honestly - I really have not had any issues with quick drying, every 2' bed was mulched heavily with the straw, and all kept pretty well moist.
Going to build 8 more 2x10 as soon as the spring will smile on us, will take a week off, and organize the garden well, add more organic matter to each bed, so those would be nice and ready for 2010 planting, can't wait!!!