The use of beadheads, particular tungsten beads, when tying nymph patterns has become popular. In respect to the use of tungsten beads, the weight factor is important in that the sinkrate of a fly tied with a tungsten bead is something like 50% greater than a fly tied with a standard beadhead. In general, depth is typically the primary factor to consider when fishing nymphs as getting the fly in the proper feeding zone is crucial to success. As such, fishing a pattern tied with a tungsten bead is going to help get the pattern in that zone.

That being said, one can fish an unweighted fly or fly tied with lead and still get it in the same zone, but it may require the use of some form of weight attached to the leader (depending upon water conditions). I would say that its tough to make general statements regarding the effectiveness of a beadheaded nymph versus a nymph tied without bead, assuming of course both are fished in the same manner at the proper depth. What a fish prefers may be different depending upon the conditions or fishery. Some situations a fish may prefer the beadheaded nymph, while in others the nymph without the bead is the better choice. There are no "universals" in this sport or absolutes.

With that, a nymphs effectiveness (or lack thereof) is associated with a number of factors, not all of them however are associated with the use of a bead. The bead can be helpful in getting the fly down to the proper depth, it can suggest the "air bubble" as previously mentioned. It may also simply be a visual trigger for a fish. These are all possibilities, but why a fish takes one pattern over another is something we can only formulate theories about based upon conditions, feeding behavior, etc..

It is best, therefore, to tie patterns with and without beads to cover your bases as in some situations, a bead can be more harmful than helpful.