>> Being Intranet doesn't mean we can assume that all the clients would be using the browser / OS of our choice
>> Intranet applications almost always run in trusted zone so popups won't be a problem unless you specifically plan on disabling them, though it doesn't make any sense as to why someone would disable an intended feature.

Popup settings aren't neccessarily related the trust level of a zone; a theoretical global setting like 'open popups as tabs' could defeat any attempts to hide the backbutton without directly 'disabling' popups, regardless of the zone; if you can't assume a browser/OS of choice, you certainly can't assume a configuration of choice.