Consumers are often wary of add-on sales whenever they purchase anything major (a guitar for example) because they are often not necessary. I can tell you, like I tell every single customer that goes home with an acoustic guitar, to please please please buy a humidifier.They are relatively inexpensive (Planet Waves makes some AWESOME ones for $15.99-34.99) and will absolutely save the life of your guitar. In colder climates such as our own relative humidity can vary from 100% down to 20% or less and most guitar manufacturers recommend storing your guitars at 45%. Huge swings in humidity can cause the wood your guitar is made out of to expand, shrink, warp, dance, crack...
Okay, it probably won't do much dancing, but the instrument you just spent hundreds on and hours playing just changed and could possibly be ruined. Sometimes adjustments can be made to allow for differences in humidity such as nut, saddle/bridge, or string tensions, but there are a few things that are generally irrepairable. A twisted neck generally cannot be fixed, which results in "fretting out" on some sections of your fretboard. A bulge can form behind your bridge which can cause intonation problems, a higher string height or in some cases the bridge will peel off of your guitar.
This is a pretty severe example of a bump forming behind the bridge, look at the angle the bridge is at!
With all of the different styles available there is one that is just right for every musician. If you are REALLY in a pinch, a chunk of clean damp (NOT WET) sponge placed in your guitar case and your case kept CLOSED will help. Check the sponge very few days to make sure it is still damp and DO NOT let it roll around in your case and come in contact with your guitar.