in my thirteen years defending skinportal with a squad of axedwarves and a squad or two of archers, this is what I have learned.
1. Even dwarves with legendary skill levels in archer and crossbowdwarf do not consistently 1 or 2 shot enemies with headshots.
2. With 1 or 2 shots they WILL consistently significantly disable an enemy, causing them to drop weapons, fall over, vomit everywhere for hours, and lose consciousness due to extreme pain.
3. They will often continue to pincushion disabled enemies. Archers do not appear to use the new aiming system. A melee troop will nearly always kill a downed enemy in one shot to the head, while archers take longer to kill. Killing shots are by no means infrequent however.
4. Squads of steel-equipped, bunker-stationed, legendary archers are quite vulnerable to even a single elite enemy archer. When said archer arrives among a group of enemies, they do not reliably obey they kill command and focus fire. However, it often only takes a single shot to disable that elite enemy archer should they aim at him.
5. This vulnerability can be significantly reduced or eliminated entirely through danger room training and shield demonstrations, the objective of which is to improve their efficiency with steel bucklers.
6. It takes years of archery practice and hundreds of bone bolts to achieve legendary marksdwarf status (or extensive use of hunting training or pitted prisoners). At this point further training is meaningless, therefore I transfer them to a second squad composed only of elite marksdwarves. This squad has no archery targets assigned, only metal ammunition assigned, and focuses its time on raising that shield user skill, again through danger rooms and demonstrations. These have the added benefit of greatly increasing the marksdwarves efficiency in melee combat.
7. Marksdwarves are great for eliminating melee enemies that are idling near cliffs or dropoffs, since there is little to no risk of them dodging off and falling. Marksdwarves can safely enter the dangerous area and maim/kill the enemies from 5-10 tiles away.
With the neigh invulnerability of danger-room trained melee dwarves, you can comfortably get by without archers these days, and you would be absolutely crazy to go without melee dwarves of any kind entirely. The biggest benefit to archers is they add new strategic options to combat.
My favorite tactic thus far has been to station my dwarves along the walls, pincushioning enemies as they enter. The unharmed enemies will enter an underground tunnel to my fortress. Then i open the main gates and my axedwarves charge out, one hit killing wounded enemies and generally taking no damage, and furthermore blocking the escape route of those enemies that had entered the underground tunnel. Enemies that flee quickly with bolts stuck in them often survive, but bleeding, vomiting, unconscious goblins often don't run so fast, and are easy targets from my melee troops.