You know CAP theorem.
Says that a PARTITIONED system cannot be simultaneously CONSISTENT and AVAILABLE.

So bitcoin network has to enforce periodically some rule for nodes to become CONSISTENT like agreeing on a common time.
Else eventually the system will always split in two highly AVAILABLE but not CONSISTENT parts (block chain fork)

Bitcoin is just fine but talking about ways to improve it by relaxing some requirements in favor of a simpler behavior (such as using some external time references widely available over the internet)  i think always helps. For example there is an ongoing discussion on increasing transactions volume