This is a very good change. You can't og arround and do everything with one stack.
I played yesterday and finally I had huge problems when the AI attacked my land With 3 huge stacks. Now more armies are needed earlier, not just one super army, or at least there are trade-offs and rewards to both strategies.
This change improves strategical planning with your armies a good notch.
And perhaps the most important point: Cavalry and infantry balances better out after this change. Especially after next patch where cavalry units will be more expensive as well.
Why? Because one costly superstack pure cavalry army will also have to end it's turn after combat, making it's movement bonus mean less compared to infantry then it used to. + Having more armies being important will lead to more importance/usefullness of pure infantry armies since they are more affordable (or infantry combined with the expensive cavalry in support roles only).
All this ties together in the greater design of the game.