(1) it gives a reason to farm (demonic essences),
(2) it takes gold out of the economy (directly through crafting costs, indirectly through people choosing salvaging over vendoring), and
(3) it results in a satisfying alternative to the auction houses.
The weapon is the most important item any character has. Since weapons are integral and crafting is beneficial, it would be a major improvement to the game to be able to craft top level weapons.
Obviously there will have to be limitations. Here are a few I've thought of:
(a) recipes only drop from ubers and are BOA
(b) require 10+ demonic essences for each craft
(c) gems are required to add stats, similar to current amulet crafting
(d) weapons can only be crafted by new artisan who randomly appears after defeating Diablo with 5 stacks. New artisan can only craft a limited number of weapons per appearance.
Any thoughts? I think it would make the game a lot more fun and rewarding.
tl;dr We should be able to craft top level weapons already.