Bicycle Mechanics - front derailleur; necessary?

Hi,
I commute on a Specialized ATB in nasty, salty, sandy, slushy
Boston winters. To minimize cleaning time I decided to remove the front derailleur since I only ever used the middle chainring anyway. Who needs 27 gears for commuting? The problem is now my chain autoshifts in front or falls off entirely (in either direction).
Not too often, but often enough to be annoying.
Is this because my chain is too long? Or is part of the derailleur
function to keep the chain on?
THanks,
Nancy

Modern chains are quite bendy from side-to-side, and with single chainring setups, can be nudged off the ring by a bump or by shifts at the rear. I use an old front mech as a chain guide on my 1x5speed bike. It has no cable, and never moves.
You can minimise chain-loss by ensuring a good chainline.
A simple ring with no pins or shifting guides may help. You can set up rings either side to contain the chain (and keep you clean). Specialities-TA made a set of rings with no teeth, for their cyclo-cross setup.

In Vancouver its smart to have 27 gears...but besides that. Look at any of the chainguides available. They will hold the chain in line and most are only meant for one chainring.

For commuting i'd go with a double bashring before a chainguide. "We be doin some hard-core commutin today eh":D

i bet it wouldnt hurt to take a few links off the chain.

Thanks everyone. I think for now I will put the old derailleur piece back on (without the cable) to hold the chain. When my chainring wears out I will replace it with a simple one without all the special shapes meant to enhance shifting. And maybe I will still need a chain guide too. We'll see. I've got to be careful removing too many links from the chain though because if it accidentally shifts to the large ring in front while I'm on the large ring in back, I could be in trouble! Originally I was going to remove the unused rings but then I discovered to my surprise that they were all welded together. Kinda annoying. Anyway, thanks again.
-Nancy

if you can't simplify things by taking off the chainrings, why bother in the first place? i would just leave the shifter/derailleur on as well, plus you never know there might be a hill once in a while?

Wheelwoman,
The reasons you describe are why it is very common for cyclocross racers to run single chainrings.
An easy fix is to keep everything as is--the big ring will keep your chain from falling to the outside. Buy a $4 chain watcher (also called the 3rd eye and a better one made by Redline). It is a black piece of plastic that keeps your chain from dropping off to the inside.
To answer your question, yes, the front derailleur keeps your chain on the ring as well as for shifting.

Hey thanks RacerX,
I didn't know there was such a thing. It sounds perfect.

I use a single BMX chainring with no guide or anything on my 1 x 8 and it works fine.

You could install a front deralleur cover.

I am guilty of doing the same thing. One hot day when I was very sick, I decided to take off my front derailleur and go with a 42 tooth front cog with 8 gears on the back 12-30, ample even in the hills. However the damn chain kept falling off whenever I changed gears on the back. So I have now reinstalled the derailleur but have left off the cable and front shifter. Now I have an 8spd bike with the derailleur being used only as a guide. I also removed the smallest chainring but had to keep the middle one attached as the 42 is bolted to that.

The moral to the story, don't touch your bike when sick.

CHEERS.

Mark