Push's PRIMARY purpose is to get information to you as quickly as possible.
Using less battery (than a constant Fetch) is a secondary consideration, and doesn't necessarily happen. A push could easily end up pinging the server just as often as a fetch does, because it must keep the network connection open.
For example, on a EDGE phone, it falls down if you get a phone call that interrupts the data connection. Mobile carriers can also have as low as a ten minute connection timeout somewhere in their system, forcing a ping more often.
Note: We cannot compare push on the Blackberry to IMAP or Exchange push. Push on a BB does not require pinging the server to keep a connection open.