A fully integrated grip allows better weather sealing, improves stability and rigidity, and can allow better placement of additional or secondary buttons and functionality than a removable grip. For those types, such as myself, that pretty much always use a grip anyway and never remove it, an integrated grip is ideal.
The real key point to having an integrated grip though is better weather sealing...you just plain and simply can't seal a removable grip the same way...and weather sealing is one of the key selling points of the 1D line.
Your comment about weather sealing is very true. I was shooting during 1 inch per hour rain in Florida with my 7D gripped, after about 4 hours, the buttons on the back of the 7D died which is usually a good indication that the 7D has been liquid damaged and is dead. However, I decided to component isolate and remove the grip, turns out the grip had died but not the 7D.
Turns out that the Canon grips are not weather sealed. If the camera/grip are in the rain, the weather sealing of the body is moot. With a permanent grip, the weather sealing would be much better.
Personally, I use a Zeikos grip on my 7D. It actually does have some weather sealing. During Spring in 2012, I used my 7D in rain, sleet, snow, and even hail for over eight hours (it was a hell of a day for weather!) There were times with both my 100-400mm lens and the 7D were completely soaked, but nothing died, and everything seemed to work perfectly. When I got home and popped out the batteries for a recharge, I noticed that the Zeikos grip actually had a little foam rubber sealing gasket around the battery compartment, and again around the extension that fits into the 7D's battery cavity. I figure the buttons are probably moderately sealed...can't say if they are as good as the 7D's sealing or not. Regardless, if you want a grip for the 7D that can hold up to the weather, the Zeikos is it (and cheaper to boot)!