Is a guitar humidifier necessary?

Yes, humidifiers are very much worth the attempt. By using one, and measuring the humidity in the room, you could save cracking and uplifting on your acoustic.

Even though I live in Louisiana, airconditioning and heating requires additional protection from cracking of the woods.

Get one. A good one for an acoustic is called a Dampit, an item that covers the soundhole with a plastic dome with a dampened pigtail hanging inside the box. A Dampit costs about $16.

Humidity is more important if you own an acoustic guitar. Acoustic guitars require a humidity of 45% to 55% at all times. Forced hot air furnaces provide very dry air. The surest way is to purchase an inexpensive hygrometer (Radio Shack sells them) and measure for yourself. If the house is too dry in winter then either humidify the room that your guitar is in or use a guitar humidifier.

I've never used one in close to 40 years of playing and never had a problem. I suppose if I had a super-expensive guitar, I might think about it, but otherwise, no thank you.

of course you should'nt. thats a dumb thing to buy. just take care of your axe.