The first and most obvious chunk of reasons that Imaginary Future Puppy probably won't be Imaginary Future Pound Puppy relate to some of my chosen sports.
I'm hoping to get into IPO/Schutzhund with the next dog. I'm a total novice to this world -- I've never trained in it and I've certainly never titled a dog in it -- so much of this is from the perspective of a newbie fumbling through a fog of ignorance.
Even allowing for that, though, I think it's particularly difficult to find a solid sport prospect in the shelter system if the sport in question demands specialized traits and instincts in the dog. If you really, really want to compete in retriever field trials or high-level herding, it helps a ton to choose a dog from a specific lineage bred for that job. At that point, you're not just looking for "a Labrador Retriever" or "a Border Collie," but for specific pedigrees and ancestors in the line.
The same is true in protection sports. Not every German Shepherd or Belgian Malinois will succeed. Your odds are considerably better if you select a dog from a lineage bred specifically to work in that area.
Additionally, for obvious reasons, most shelters are not real eager to adopt out dogs who show signs of being willing to bite people (even in play, which is the foundation of most IPO training -- the dog is taught that the "bite" is just a great game of tug), and the ones who do have that potential are likely to come with a host of other issues that I'm not necessarily equipped to deal with.
Many of those "issues" might be no more than typical high-energy adolescent rambunctiousness, but the fact remains that I do not, personally, want to deal with an untrained adolescent working-line German Shepherd or Malinois at this point in my life. This'll be my first working-line dog of a "serious" breed, and I really think I'll be best served by raising the pup from, well, a pup. Starting with the 80-pound version instead of the 8-pound version is probably more than I can handle.
One of the big pieces of conventional wisdom that IPO people like to emphasize is to be very careful about getting more dog than you can handle, especially your first time out... and while I'm sure there's a certain amount of scare-the-noob hyperbole in that, there's probably a fair amount of truth, too. Particularly in our situation (tiny condo, no yard, neighbors on all sides, two other big resident dogs who were here first), throwing a full- or mostly-grown male working dog into the mix seems like a Bad Idea.
Another factor that's more or less specific to IPO Land is that some clubs and training directors are very resistant to allowing "off breeds," let alone shelter mutts. From what (little) I've seen of the IPO world, it's not too welcoming to unusual breeds and unorthodox teams. Some clubs won't accept anything but German Shepherds, and some won't accept anything but Malinois. Even breeds like Dobermans, Rottweilers, and Bouviers are often disparaged as unable to do the work. The belief is that even if these dogs can participate in low levels of the sport (and, honestly, many can't), they'll never be "good" at it, so therefore they aren't worth the club's time or the wear and tear on the helper/decoy.
This isn't a universal sentiment by any measure (there are also plenty of people really excited to see "off breeds" succeed on the field!) but it's common enough to be a factor. Breaking into a new sport as a total newbie is hard enough as it is, and breaking into this particular sport as a force-free handler is even harder. There's only so much uphill pushing I can take. So when it comes to this particular issue, I'm out to make my life as easy as possible.
So all this leads to the question: If I didn't want to do IPO, and all my other goals were the same, would I be open to considering a pound puppy?
I might. All my other reasons weigh a lot less strongly. Which is funny, because I'm not even sure I will do IPO -- I might just try it briefly and drop out after two weeks -- but I want to keep the door open. Even if that means closing it to pound dogs.