[MENTION=3987]Bagpuss[/MENTION]: the point of putting a finite number of actions in combat is to add simplicity and logic to the combat experience. In the 3E system, a character can literally dodge an unlimited number of attacks, provided his rolls are good enough, in the course of a combat round. This combat round then gets more complex with a different type of defense, called Saving Throws, which can also come in basically unlimited numbers. Throw in swift, full round, movement, standard, and free actions, plus opp attacks, and after one combat round, you're lucky if you have an idea of what just happened.
So to answer your question, "why wait to attack?" the answer is: a combatant considers defense a worthy endeavor. Now, if an "opportunity" arises which makes attacking look better than defending, why not take it?
[MENTION=40857]Meatboy[/MENTION]: a valid observation. Sometimes the best defense is a good offense. Sometimes it's not. But a three-action system allows that choice to be made, no?
[MENTION=83293]nnms[/MENTION]: I was actually thinking that using a bow would require a reload action, crossbow two, etc. Further, I don't see why there wouldn't be ways to get more actions, especially with the improvement of a character's skills or abilities.
[MENTION=6746469]Dethklok[/MENTION]: As above - I'm not going for simulation; I'm going for simplicity and logic. A standard-action-only system eliminates the need for movement actions, full defenses, and specifically, attacks of opportunity. You don't need a feat called Agile Riposte - if your opponent attacks, and you have an action ready, you can choose to block or begin your counterstrike. If you've spent your whole round doing other things, then it's likely you simply don't have the time to block yet another attack.
My sparring experience is mostly with fists, so please let us know where you're going with the question.
I see a bad outcome to a finite-actions combat system: it makes being outnumbered a very bad thing. But is that really out of the ordinary? Is it something a hero (or heroes) can't handle?