[JKCalhoun]

Yeah, to me it is an artistic thing (then followed perhaps by painters copying "the Masters").

Fingers spread evenly is artistically uninteresting — naive even. Fingers all joined is also rather dull — suggests a rigidity in fact.

[kmoser]

My thoughts exactly: by bringing two of the five fingers together you achieve a more interesting effect of overall asymmetry among the fingers, and yet because the two joined fingers are each flanked by a "detached" finger, it creates another layer of symmetry within the non-thumb fingers.

The overall effect is quite pleasing to the eye, which may account for it having caught on to the point where it became a trend. I see this as the Occam's Razor of explanations.