I used to work for an airline where the F/O was not allowed to taxi and now work for one where we are allowed to taxi.
From a personal point if view, I feel that it gives me a a greater sense of responsibility when I am P1 for the sector.
It's me who liaises with the dispatcher, it's me who liaises with ATC, it's me who liaises with the ground crew, it's me who is in control of the movement of the aircraft, both in the air and on the ground. It's me who runs the show.
At my previous airline, landing back at base, when the Captain took control as we hit 50kts on the landing roll, my responsibilities were over for the day and it was very easy for my head to already be in the car park.
Like has been mentioned, if we as an F/O are trusted to take off, cruise and land an aircraft, and, let's not forget, do all these things if the Captain were to become incapacitated, why can we not be trusted to taxi the aircraft?