(a) The orb can make things considerably easier, however unlike a lot of powers that eliminate someone from the fight temporariliy, this does not damage those it eliminated (i.e. those it didn't take into the orb). It will make 1 encounter per day easier ... of course, most daily powers make an encounter a lot easier than it would normally be.
(b) It is possible to use "waves" as a way to make a party able to face higher level encounters than they would normally be able to do. That was the idea behind the Irontooth fight, and the orb can be a way for the DM to up the challenge level against the party. Having the monsters that were "left behind" get reinforcements isn't completely out of the question, especially if they are fighting intelligent creatures.
(c) Skirmisher/Lurkers are the monsters that will really hate the orbs, artillery as well to some extent. At the same time, brutes and soldiers won't care that much. I think that's been mentioned a few times already. They aren't nullfied, but they are definitely hurt. At the same time, the same can be said about just about anyone but the defender and melee leaders ... the ranger/warlock/rogue is going to be limited in their mobility [the barbarian will have a hard time charging], and the wizard is forced in close quarters, not to mention he has to pretty much avoid doing any area of effect attacks while inside the orb.
(d) There is no mechanic for disarming per se in the first place. The rules for things like "what happens to stuff you leave in the orb" is likely something that can be handled by the DM. The easiest way to handle things ... anything not "native" to the orb is shunted out at the end, whether it's held or not. So whatever was disarmed would be in the same relative space it was when they got out. If a DM thought it would be cool to have the PCs build a club house there (over the course of many days, 5 minutes at a time) he could, but it does seem a bit implausible. Also, if they bring in that much material, then it would likely start pulling in material when the orb is used, making terrain features from one area come along for the ride, which is probably not the normal use for the orb.
Ultimately, I'm not saying everyone should allow the orb. They put the disclaimer there for a reason, these orbs aren't just run of the mill magic items, they are "allow the players to have them at your own risk". What I am saying is that, a DM that gives them to the players it's doomed to failure if they understand that going in, and design their encounters accordingly. The players, by that point in their career are going to have a lot of powerful abilities. If the DM just designs encounters without factoring in the PCs abilities, they are going to have their plans foiled quite often.
Ultimately, if returning from inside the orb just means finishing what's left of the encounter ... and that is what always happens, then the DM isn't really trying. When the ENTIRE party disappears, depending on how it is, I would think the other monsters would act. Either the party ends up coming back and ending up face to face with another encounter that has shown up to help, or to find that the rest of the encounter has fled, perhaps to increase the population in a different encounter.
A DM that sees it as a prolem doesn't have to include. A DM that sees it as an opportunity can work with a party that has the "rare" magic item. Regardless, the item should never be considered something that is easy to make/buy, and should be kept away from artificers ... but then again, both the orbs AND artificers are in playtest, so that particular situation way be solved before they both become official.