To put it another way, consistency for consistency's sake alone is  
not the goal.
Consistency helps make things somehow predictable, but sometimes it's  
just bloat.

That said, if Ruby had immutable classes, it would be important to have
push		# return a new Array object with the result of a push
push!	# alter the Array object in place with the result of push
pop		# return a new Array object with the result of a pop
pop!		# alter the Array object in place with the result of pop