This is a tricky issue. The airline may not let you board your return flight to LGA if you skip DFW-HOU-DFW as Jetsetter correctly pointed out. If it was the last segment of your itn. then there wouldn't be a problem but right smack in the middle might cause something unexpected.

There are ways to get around hidden city rules and I just read a post today about a travel agency that does it very skilfully.