I think the majority of the people that don't like the grip/beavertail safety are thinking about "what-if" scenarios and to dismiss those scenarios is ignorant and naive. One example would be what pausekey mentioned about one hand being out of commission. What if it is your strong hand and you have to cross draw and fire with the weapon upside down? I can also think of a few more examples. On a quick draw in an intense situation, if I were to get a not-so-great-grip on the gun, I would rather be able to fire a few rounds, readjust my grip if need be and continue firing rather than nothing happening when I pull the trigger, readjusting right now and then being able to fire. The difference in putting a couple of bullets in the targets direction might be the difference in the fight. If those rounds land on target, the fight may be over, and if they don't land on target, they might be suppressive enough to buy you the time you need to gain a positive grip. Granted since 99.9% of most people shooting will be in a controlled environment, you may not ever need to worry about that, but some people do actually rely on their weapon as a survival tool. I have noticed that sometimes when I am locking the slide back to field strip my XDm, I don't get the grip safety fully depressed (I hold the pistol sortof sideways in my shooting hand when I am just locking the slide back) and have to adjust my hand to get the grip safety down to lock the slide back. Now, as this will never actually happen in a firefight, it is a negligible detail, but still a detail nonetheless. Some people love the grip safety, I honestly could do without it and if the XDm were offered without it, I would have definitely chosen the model without it. I have read people opinions on the benefits of a beavertail safety, but I disagree. I think the whole reason they are still put on weapons is because way back when it was invented, it seemed like a good idea (which it wasn't any better of an idea back then as it is now), and since weapons with the beavertail safety gained cult level popularity, it stuck. Now people are afraid to bad mouth it b/c they think they will get ragged for saying their is a fault with the 1911.
By the way, I just got my XDm today, and haven't even shot it yet. I am going to Coal Creek tomorrow to try it out, but that being said, I have shot XD's (no XDm's yet) and plenty of 1911's with a grip safety, and everything I mentioned is a possibly failure point for the weapons I have shot with a grip safety.