What are the drawbacks of placing time limits on turns?

I personally find general concept of enforcing time limits on player turns during combat a very, very good thing for the following reasons:

  • Just like in a real battle, you don’t have a lot of time to think. Time limit represents this quite well.
  • You don’t have enough time to get distracted until you are acting again, no time is wasted while you think of something else, check Facebook feed etc, because waiting for everyone to finish may get boring.
  • You can go through more encounters per hour, combat doesn’t take forever to resolve.
  • Combat becomes much, much more intense.

Etc, etc.

How I suppose time limits to work:

  • You are only allowed to talk during your turn. That includes no asking for any clarifications, tactical advice, discussing what goes on while it’s not your turn.
  • While turn of the player who acts before you starts, GM calls out your name and says that your turn is next.
  • When that player has acted and math is applied, GM describes what has changed. You are supposed to listen carefully, GM doesn’t describe what’s happening for each player individually. Sometimes it can take 10 seconds, sometimes minutes — doesn’t matter.
  • Your turn starts, you have 30 seconds per action your character can do. During that time you are supposed to tell the GM what are you doing and roll dice. Math doesn’t count. If you need any clarifications, you have to use those 30 seconds. Same if you want to say something to anyone in-character during combat. Once you roll, you stop talking, GM counts and announces the result of your roll and current situation.
  • If you don’t both describe your actions and roll dice during those 30 seconds, which most likely means that you didn’t have any plan in your head, you make a “default action”, which is decided in advance.
  • If you only described what you want to do partially and your time ran out, you have ~3 seconds to say if you do what you had time for to describe, if you perform a default action of if you do nothing.
  • Of course, people may ask for a game to pause if they need to bring some tea, answer phone etc. After all, such intense combat may get people tired.
  • GM describing what goes on between your actions doesn’t count.
  • Taking a full-round action has to be described during your first 30 seconds. If you don’t do it, you cannot perform a full-round action that round, and one of your actions is lost as usual. Though, you might (in advance) name any full-round action as your default action.

The only real problem I see is that new players need more time to think anyway, sometimes even some help from others, but there are some experienced players who object to such solution.

I am myself new to Pathfinder E6, which I am going to use this solution for, so answers related to this system are most welcome.

What are the drawbacks of setting time limits on turns?

I expect answers stating exact problems that were caused by time limits actually observed during gameplay.