Are you sure?
That and the fact that you could sit around scouting, and no one knew you were there. Except the enemy infiltrators.
Infiltrators who hope to do cause real change, and affect the political status of the game world, will not hit enemies. What purpose does hitting your enemies serve? They're already your enemies. The way to cause real problems is to nail someone you wouldn't be expected to hit, and pin it on someone else. You can't do that if you're invisible.
I agree that the infil game needs some changes. Anaris has already outlined that this is on the long-term plan. But simply adding back invisibility is not the panacea that will suddenly make infils relevant.
So what about scouting? Even better! At least, when the enemy knew what you were doing, you could suspect "damn, they've got infils planted in position" instead of automatically going hyper-paranoid "WHO IS LEAKING OUR ORDERS!?!" Gave plausible deniability to your realm-mates, if you will. Indeed, scouting was a form of low-risk infil activity that allowed infils to make themselves useful after having spent months training infiltration, but before taking more months to train swordfighting.
The uses of striking enemies are numerous and need not be listed. They may not be the same as the ones you describe, but the game doesn't actually favor that. If hitting friends and allies was intended, then why does it not yield any training? Risks are higher and rewards are lower. It also doesn't go much in the teamwork spirit to go around stabbing one's peers. I do not believe that this was ever the intent behind the creation of infiltrators, even if that may be how to achieve the most impact.
Invisibility is indeed not the panacea, as you put it, but there is, in my mind, no denying that its removal hurt infiltrators and the game overall.
