BlackBerry push email is addictive. Most folks don't "need" it, but you get used to it pretty darn quickly.
Practically speaking, if somebody needs you urgently they should call. If they need a response "soonish", text. For email, plenty of folks don't even have access during the day so sending an email and expecting an immediate response is unrealistic. However, in business if you get the email before your competitor gets it, say for a customer inquiry on a project, that lead time can make all the difference.
The driving concept behind BlackBerrys is to put Outlook in your pocket while you're away from your desk.