I don't get excited about fishing egg patterns like some fishermen do. But, there's good reason for their excitement: they work. It's a simple fact. When spawning salmon are in a river dropping eggs, other fish are eating them. In these situations, fishing an egg imitation will almost invariably out fish any other method available to the angler. That angler typically has two options in the world of fly fishing: the more traditional yarn-based egg patterns such the Glo Bug or the more recently adopted plastic bead egg imitations.
Beads, however, are considered a dirty word by some fly fishermen. Most commonly, bead detractors will claim that "beads aren't flies", typically because they're made of plastic, not hand tied and so on. Essentially, those who would sneer at the idea of fishing a plastic bead are making the argument that glo bugs, eggs tied with antron (plastic) and other similar yarn-based egg patterns represent a more purist, creative presentation of an egg imitation. Anyone who has tied an egg pattern—which takes about 10 seconds even for a scandalously unskilled fly tyer like myself—should quickly dismiss this as nonsense. More important, however, is the reality that anyone that has fished both beads and egg patterns should quickly realize many reasons why beads are considerably superior to their yarn-based alternatives.
Beads are More Realistic
Beads, practically inarguably, are a better imitation of real eggs than an imitation made of yarn. Beads are available in an almost endless array of colors and sizes, in varying degrees of opacity, with and without mottling, with and without blood dots and so on.
Your typical bead is about one-tenth to one-third the price of an egg pattern, if you're going by typical fly store prices. If you tie your own flies, this difference will be smaller, but you're still going to be paying almost as much for a bare hook as you will for a bead. When you consider that egg patterns catch lots of fish and thus take lots of abuse, this difference adds up quickly.
Beads are practically indestructable, at least during exposure to the normal rigors of fly fishing. Yarn egg patterns, on the other hand, don't hold up very well. Rocks, fish teeth, branches and other in-stream hazards shred yarn egg patterns easily, making them lose their shape and profile, rendering them unfishable.
More Natural Flotation
Many plastic beads are designed with neutral or near-neutral buoyancy in an effort to mimic real fish eggs. This means that when they float in the water, they do so the same way that natural fish eggs do. The result is a more realistic presentation, which equals more fooled fish.
Significantly Lower Fish Mortality
The evidence for this is anecdotal, but considerable. If you're pegging your bead an inch or two above your hook (which you should be), fishing beads normally results in a jaw hook set as opposed to a swallowed egg and a deep-hooked fish. Trout and other fish that are gorging on eggs are doing just that, gorging. Eggs are taken with abandon and are often swallowed. If your hook is trailing behind your egg imitation, setting the hook will more reliably result in fish that are hooked in corner of the jaw or elsewhere in the mouth, instead of down deep where the chances of a successful release are greatly decreased.
Finally, whether you're fishing an egg or a bead or any other fly, you should never target spawning fish and always take care to know how to identify redds and avoid trampling or disturbing them.