IPv6 ISATAP Doubt

I just have a few questions about IPv6.
Hope it isnt too newbie :roll:

In Understanding IPv6 book, it is said

ISATAP allows you to phase in the native IPv6 addressing and routing capability on your intranet in the following way:

■ Phase 1: IPv4-only intranet In this phase, your entire intranet can be a single, logical ISATAP subnet.
■ Phase 2: IPv4-only and IPv6-capable portions of your intranet In this phase, your intranet has an IPv4-only portion (the logical ISATAP subnet) and an IPv6-capable portion. The IPv6-capable portion of your intranet has been updated to support native
IPv6 addressing and routing.
■ Phase 3: IPv6-capable intranet In this phase, your entire intranet supports both IPv4 and native IPv6 addressing and routing and ISATAP is no longer needed.

Why cant I just directly go into Phase 3, why use ISATAP? In windows server 2008, isnt it possible to setup both IPv4 and native IPv6 together?
How do IPv6 PCs and ISATAP PCs access IPv4 resources off the IPv4 Internet?
Why would a company want to use ISATAP if they could enable native connectivity on the PCs directly? Just wanted to know :

Thanks in advance