I can see how it could rattle a tennis player. These guys are pros, they are in the zone. When they are on the court, it's them, their opponent, the ball, and the official. Just pure business. You're so focused on playing these mind games with your opponent, and trying to figure out what to do next, being as precise as possible. Then, out of nowhere, a kid jumps on the court and starts running towards you. Yes, it was after the match - I get that. But surely most tennis professionals would feel violated of some sort, and certainly this could play with your mental state the next time you enter the court.
Player safety is one thing, and of course the most important part. But personally, I think this would be a bother because mentally, you may not feel as secure and alone on the court.
Federer has all the right in the world to be upset, as I would be too. It's pretty disturbing how easy it is to get on to the court, but even moreso, how slow security was to react.