If you expect a dog to behave perfectly while on a leash, walk at your side, mirror your actions, heel upon command, etc., a retractable leash provides both the owner and the beagle with the temptation and flexibility to pretty much destroy the routine and predictability necessary to keep the well-trained dog behaving like a small robot. The long tether also provides enough space for the dog to suddenly dart out into the path of an oncoming car or veer off to scare the neighbor kid walking home from school.
Of course we're discussing beagles here, so with that said, the usual dog walking logic changes just a bit. One of the reasons we have beagles is because of their happy-go-lucky personalities and the way they bounce from one exciting thing along the trail to the next. In other words, I really don't want our beagle walking with me in lockstep fashion as many dog trainers suggest. Instead, we keep our beagle on a short leash when it seems prudent and give it more flexibility to romp, play and be a beagle at other times. And a retractable leash is perfect for this. With a little extra effort and training, you and your beagle should do well at both.
When giving them, say 20 feet of freedom, however, it's extremely important that you still retain control of them while always anticipating problems. If you're on a sidewalk, reel them in when a car, or another dog or a pedestrian approaches. They'll also have more opportunity to get tangled around bushes, dart into the street, duck under parked cars and get themselves in various sorts of trouble, so it's important to pay extra attention.
If you're like us, and like to take your beagle on hikes in the hills and woods, retractable leashes are difficult to work with because the long, thin leads invariably get tangled up in brush. For that we usually use a 10-foot nylon leash, then switch over to a retractable leash in more open areas.