First of all thank you for your input, scausi.
I already found that explanation over the net but I'm not persuaded.
Actually, Russians are still making finished
grips that way (for example: http://snipurl.com/27j36sk
Excluding the climate-hand swelling issue, I can think of three explanations:
1. The large "wings" are unnecessary (but is it true?). Those on russian ones are simply a matter of fashion and that's it.
2. Thinking the other way around, Morinis and Pardinis stock grips aren't finished grips either (finished meaning taylored to shooter). So, I wonder if they are left intentionally open to allow adding more material over the back of the hand if desired. This can go along with
3. Costs. These modern style grips are machine made. They benefit from a simpler shape that requires less wood and probably less work to be made.