In the Manage phase of the Daily Workflow, we explicitly advise that you act on each message quickly or convert it to a task or appointment and file away the original message. The idea is to process the message when you first look at it so that you can get it off your mind and out of your Inbox and you don’t waste mental cycles dealing with it. There is one expert-use case where this doesn’t make sense.
When you encounter a message that you know you want to respond to today, but you either need to think on it a bit or you don’t have time to answer right now, it doesn’t necessarily make sense to convert that message to a task. Flag this message via a right click so that you know that you looked at it and you’re waiting to process. Then, at the end of the day, go through each flagged Inbox message to answer and file.
"If the reason you are not responding is there is some other task-like action needed first to fulfill the request, then better is to convert the e-mail to a task, and plan on working the task, and then reply immediately saying ‘I need to do xyz before I can get back to you on this; I will get back to you as soon as possible’ or some such reply."