or just make the orb affect only one saving throw, so it is like the other implements that affect only one roll.
Orb of Imposition
does only effect 1 person. The "Save vs I win" combo that Stalker described requires the following:
1) At least be 11th level as Spell Focus is a Paragon feat (and adds -2 penalty vs saves)
2) Have a High Wisdom score...which depending upon your Stat Generation method, will impact other areas of the game, ie Low STR, CHA etc
3) Requires 2 very specific Magic Items from Adventurer's Vault.
Which means that if you play in a group where in general, the players do not: make their own items, buy every splat book to scour for every possible advantage they can find, and take treasure as it is given to them......this problem will not really arise in your game.
If you play with a horde of Min/Maxing Powergamers with an abundance of disposable income...then you might want to plan for it.
The sad part is the system itself almost requires this level of penalty stacking....when Solos have a +5 bonus to saves, you best have quite a few stacking penalties to even bother to use a Daily with an effect based off a Saving Throw on a Solo. The flip side is the Solo will probably have a chance but the "yard trash" with him will fail.
It is also an Artefact of the system, that a Level 5 Daily has an effect that is unique enough, that conceivably a player may keep that power as opposed to trading up to higher level Dailies.
Stalker, if I may ask, how does your DM contend with you "Save vs I win" combos, or has he or she put a hit out on you already?