An  ounce of prevention is worth three hours of striping,  sanding, and repainting.

It has to do with an effect called "adiabatic condensation." A small amount of water can lead to a major blarch in your paint job. 

I'd say yes, without a doubt. They are inexpensive and easy to install. While you've not had a problem so far, that can change in one painting session. Given the havoc that a tiny amount of water in your paint can cause to a model you've worked hard on, it just isn't worth the risk in my mind.

You can buy a good water trap for about $20.00. A few dollars for fittings to make it all work will ensure that you don't have water in your line is money well spent. I have a water trap mounted to my compressor and an in-line trap on each hose. I think it's well worth it.