How? One of the basics of Wicca is: "All goddesses are The Goddess. All gods are The God". That's Wicca 101. Take it away and you no longer have Wicca.
Wicca is *far* from homogenous. That's one theory on deity, and it's not held by all Wiccans. I believe you may need to read a few more books, and talk to a few more Wiccans.
In my own coven, there's folks who look at deity from a more Jungian approach, and those who feel the Gods are separate entities unto themselves. I have never seen my growing movement towards hard polytheism to be in conflict with my practice of Wicca. Perhaps the religion started with most folks working with the same God and Goddess, but it seems to work easily enough with many different deities invited into circle.
I won't keep going on, since this forum isn't really here to focus on Wicca. I'll just comment that I think the ability of most pagan paths to function properly for the adherent no matter what his or her current working theory is on deity is nice. Perhaps because so many pagan paths are paths of orthopraxy rather than orthdoxy. Certainly makes being a part of an eclectic group a lot easier!
Recently, I've been working on adding some more "authentically celtic" practices into my Wicca. It's an evolving process working between Wiccan ritual form and ADF-Druidry ritual form, with Irish invocations thrown in. I'm still fine tuning it. Haven't quite gotten it cooked down to a nicely workable format yet.
Karen