WoWGrrl explains that PUGs are an excellent way to add new guild members to your roster, since for the most part they consist of people who have never grouped before running an instance together. I know from personal experience that much of my guild recruitment in the early days was based in PUGs: a few of us would run an instance, find a player with a great sense of humor who knew their class well, and we would invite them to join us. If we were charming enough as a group, proved we could work together, then every great once in a while that person deigned to join our guild.
Once you're in the guild, however, the tendency is to try to get guild-only runs. After all, you joined the guild for support, right? Only a guild is made up of people of varying levels and interests, and while sometimes guild runs will fall magically into place, more times than not you're left feeling like guild chat is your own worst version of the LFG. This is where the PUG comes in. Where the guild might only be on at certain times, looking to work on specific tasks, a PUG is more consistently available. I say this because mathematically, there are more people who are out of your guild than people in your guild.
Reading WoWGrrl's discussion has reminded me that PUGs can be useful tools even when you are in a guild. Heck, they even strengthen the guild itself when done consistently. I think I'll log in and start a recruiting PUG of my own.