I don't look at it from a realism perspective. I look at it from a gameplay perspective: everyone should have access to all the information of their own army, and the general should have access to that information for all armies. Battles would still need to show actual CS and I do believe you should be aware of the CS in your current region as well. However, showing a certain randomness in scout reports (you can still tell the strength of an army by looking at it's numbers, just compare it to your own, that's how everyone, everywhere does it), which is actually already something that's going on to a lesser extent today, does add gameplay. Not only does it give more worth to training and to RC quality but also actual tactical advantages to those who have put time, effort and gold into it (as is with everything in this game).
Just imagine: a realm invades you. They send 1,000 men. You have 1,400 men of medium quality troops who have 16,000 CS. You estimate their CS based on what you know of their RCs, the time they've had (you've given them) and/or previous encounters and then you decide whether to engage them or not.
You'd think that you should be able to beat them at those odds, but maybe those are 1,000 killer troops, diligently trained and recruited from RCs that have taken tens of thousands of gold to perfect. Or maybe it's simply all your enemy could muster.
The point is, you're not just mindlessly comparing CS to see whether you can or can't win a battle, adding or subtracting for fortifications. You're thinking about it and learning from your enemy. For the first time ever, to fight a war you would have to find out AND take into account the quality of your enemy's forces when devising a strategy. Strategy would be more than just getting the biggest number in the right place at the right time.
It would tie military operations in nicely with everything on which you also spend thousands upon thousands of gold just for the purpose of those same operations.