Depending on the language, that trailing comma can kick you in the nuts... since it can create an extra empty array or object element. That's a waste of memory, and can result in performance issues since you're allocating more RAM... much less what about telling how many ACTUAL elements there are?
In other languages it won't even make it past the compiler or execute stage...
So why the *** would you be doing that in the first place exactly?!? "oh noez, eyes bee two stupids two addz uh comas laters?"