Geas isn't some unbreakable compulsion; a remove curse from a sufficiently capable spellcaster will work, as will a limited wish (to say nothing of a wish or miracle).
Plus, with the 10 minute casting time, it's not exactly easy to cast on unwilling targets. To have the spell cast on you against your will, you basically have to get beat up first; and if you've been smacked down that hard, you should've been dead. So an unwilling geas will most often be an alternative to a TPK -- gosh, I trade temporary servitude for a level and a load of cash (assuming I could get raised at all)? Sign me up.
Plus, what's the big deal about just ignoring the geas? Ooh, you take 3d6 damage per day. That's an average of a cure light and a cure minor wounds per day, per person. The sickened effect is tougher to remove (just heal, in the core rules, AFAIK; ISTR there being a lower level non-core or d20 spell that removes that condition, though), but there is a Fort save to avoid it, and it's just a -2 -- it's not like you're suddenly helpless. Apply liberal doses of carefully timed bear's endurance and similar Fort save-boosting spells, and take your chances.
Per the SRD, geas "functions similarly to lesser geas, except that it affects a creature of any HD and allows no saving throw." Lesser geas has a 1 day/level duration; I don't see anything in the description of geas that overrides that, so geas also functions for 1 day/level, max. Barring epic level spellcasters, that's 3 weeks, tops.
(And a lesser geas is a joke for any group with a cleric of 5th level or higher; take the -2 to ability scores for the day it takes the cleric to prepare some remove curses, and be done with it.)
As PC arm-twisting techniques go, geas is pretty weak.