EDIT: Naxte, you are assuming that CAP10 takes absolutely no damage from attacks whatsoever. When does any pokemon ever stay in for 9 or 10 turns without being attacked by the opponent (unless we're talking something like Rotom-H vs a last poke CB Scizor stuck on Superpower)? The 10% that is lost from Life Orb can make the difference between life and death for CAP10 in far fewer turns than 9 or 10. My issue with Magic Guard is that it allows for a free boost to attack and special attack, since Life Orb's boost only has the price of 10% of CAP10's health.
That's pretty much my point--CAP10 won't just be taking 9-10 attacks straight on, without being attacked back--it
will be attacked during that period, even if it does keep switching in and out. Due to that, the Life Orb recoil not being there doesn't seem too significant, as CAP10 isn't going to be surviving that much longer due to the fact that, as you said, it will be attacked in that period. As a result, it might get off one,
maybe two more attacks off at most, but since it will be getting attacked regardless, the fact that Life Orb recoil isn't there will most times not really be significant, as the same EQ or whatever that killed it without Magic Guard would still most likely kill it. Magic Guard may help a bit, but due to the fact that it will be attacked and not actually get 9-10 attacks in either way, it doesn't seem too significant to me. In any case though, it won't make it be a significant boost to its sweeping ability, so, IMO, while it of course boosts its sweeping ability, it's not such a significant boost to its sweeping ability compared to what it could do otherwise that we should really be concerned about it.
And of course, that was only part of my point. Again, the alternative is Poison Heal, an ability that focuses on increased healing. If we can handle that, we can handle a Life Orb without recoil, IMO.
And even with Magic Guard, Life Orb isn't exactly a free boost, as was one of my other points. Magic Guard doesn't let it get any stronger (stat-wise) than it could otherwise--Life Orb is still an option either way. It also still has to give up its item slot to Life Orb; it can't use both Leftovers and Life Orb at the same time or anything. But in any case, whether that's true or not, that just ends of going back to just how significant the recoil not being there will be. And IMO, it won't be that much of a difference. But, this is just theorymon, really, regardless. The only way that can actually answer this sort of question is to try it and see what happens, so I won't press this any further than that.
Edit: And I forgot to mention this: it's not like Magic Guard doesn't have it's own drawbacks, as well. Unlike Poison Heal, you can still paralyze CAP10 with Magic Guard, which it really doesn't appreciate. You can also burn it, which still hurts its attack, and Sleep it. With Poison Heal, unless you do that the first turn its out, you won't be able to. So even if Magic Guard does tempt CAP10 to be more of a sweeper, in addition to still having to deal with being attacked in return as it tries to sweep, it would also still have to deal with Burn and Paralysis, neither of which it would like. So again, I'm not terribly worried about Magic Guard in this respect, but this is really something that can only be resolved by actually seeing it in action and observing what happens, IMO.