I've heard of clans merging with others to form specialized divisions within a larger umbrella, though I personally have never experienced that. In this case, it sounds like you advocate the opposite approach, with a warring clan (let's take iMag as an example, as it's readily available) gathering a bunch of smaller clans under its umbrella so that they can net while iMag acts as the retal/killer division.

The problem with this theory, I would think, is that iMag can't protect everyone. With the smaller clans/untaggeds being farmed by all netting clans, iMag--a single clan developed for a completely different purpose--wouldn't be able to fend off ten clans at once. It would be a wasted effort, and in the end might hurt iMag membership when members decide they've had enough of this "protection" idea and move somewhere else.

The difference between a warring clan and a protection umbrella is that the war clan looks to fight a limited number of enemies and leave the rest of the server alone, while the umbrella has to look out for threats throughout the server simultaneously. A war clan is simply not built to do that job.

I personally do not play, nor have I played, in a war clan, so I could be inaccurate on a few points here. Please correct me if that's the case.