Responding to every message/request with a "I'll get back to you by X" takes you away from the task you were otherwise doing. This may not matter, but it's often yet another diversion. Most of us are paid/valued on the basis we can do sustained work and responding to adhoc queries is only a small part of that.

I think there is a peril in responding immediately (which I tend to do) - you set a weird informal SLA that people then expect you to meet. I've had people call me and say "Hey I emailed you this morning - are you ignoring me?" - they're kind of joking but not really.

It's tough to balance. I agree with you - sometimes, me spending five minutes not only saves someone else an hour, it saves me hours later on when I have to unpick something someone else has done. The skill is trying to identify those critical points without interrupting what you're doing every time you get a mail/IM/whatever. A good team helps with that.