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Recently I've been watching the KBS Cup game videos. I haven't measured the main time in this tournament (and I don't know enough Korean to read the announcements), but the byoyomi appears to be 30 seconds. One thing that surprised and enlightened me is that many of these pros really used up 25-29 of their 30 seconds on almost every move unless it was answering a kikashi or playing some common endgame sequence or joseki. In a casual game they could of course play much faster, as we can see when the players review the game (which is also shown.)
The question of how players spend their time when not under pressure is not of much interest to me---I think that varies from player to player and game to game as is shown by this page---but how they use their time effectively when under pressure is important.
Online, I'd developed a bad habit of playing too fast for the time limits. When my opponent speeds up, I speed up, too even if it's not necessary. And I certainly don't use most of my byoyomi period, but that's partially because I've lost too many games due to net lag.
"One thing that surprised and enlightened me is that many of these pros really used up 25-29 of their 30 seconds on almost every move"
They are professionals so naturally they have learned to play the clock and make the best use of available time by looking at other parts of the board.
You will also see pros occasionally play a ko threat to gain more clock time. In this case it is common for their opponent to play quickly so as to allow the time crunched player as little additional time as possible.
No matter how byzantine the time system you can be certain that top players will train themselves to use the time system as effectively as possible.
From a spectator and commentator standpoint I think Fischer Timing, in addition to being better for the players, would give the spectators and commentators more information about the player's current mindset because the players would store up and release time reservoirs according to the complexity of the position they face.
Yes, one also sees typical "byoyomi" moves such as the wasted ko threats you mention. I agree Fischer timing or some similar non-spilling scheme is superior, but the question currently seems academic. For the forseeable future, the majority of games I play in face-to-face tournaments will be Japanese byoyomi style, and online it's often hard to get a game with non-traditional timing systems.