1) Katana has ... running speed
This great when you need to kite around so you don't get boxed in (speed is what make medics so great at kiting). Speed is also great when you need to rush a siren or a bloat and take them out before they hurt - without falling back on team members or a ranged backup weapon. Or when you want to sidestep a crawler and then kill it after it land without taking damage.
2) ...the Chainsaw's secondary attack...
Then again - Katana primary attack stun scrakes. It is faster than chainsaw secondary attack (that also stun scrakes). This let you backpaddle while still keeping scrake from attacking - using primary attack from the katana...
If katana is 5% stronger than a chainsaw for non-berserkers it will still be 5% stronger than a chainsaw for level 5 berserkers. That is also simple mathematics...
4) ...Why would you trade ... for a ... weapon that allows you to hold one extra ... weapon...
Katana let you equip crossbow. Crossbow one-shot kill multiple crawlers or sirens, even when they are behind other zombies.
Chainsaw is better when you, and your team, hold your ground at a doorway/staircase while zombies zerg you - think the u-shaped room in bioticslab or the upper side of the middle staircase on office. It also decap fleshpounds that are after you easier than katana (but that it is only a matter of practice). Crawling around with the loud Chainsaw is boring but efficient.
Katana is better when you, and/or your team, is on the move - on open maps - think london or farm. The speed you have when wielding Katana is better when you hunt down an enraged fleshpound or scrake going after your team members. Running around with Katana is fun and efficent (this is perhaps the strongest argument to use Katana as a Berserker)
AND - as a berserker you can buy BOTH, even at up to 60% discount. This let you switch between them so you can ignore weaknesses and utilize the strengths of both weapons
As I see it you have quite a few viable combos:
- Chainsaw (when holding ground, being swarmed or need to decap fp) + Katana (for everything else. You must have room and time to charge/sidestep sirens/crawlers - or take them out with 9mm).
- Chainsaw + handcannon(s) (for crawlers and sirens)
- Katana + Bullpup (decap fp) + handcannon(s) (for crawlers and sirens when you dont have room to sidestep/charge them)
- Katana + Crossbow (one-shot multiple crawlers and sirens even if behind other zombies - at later waves you need to know how to one-attack-decap fps)
- Only Katana (4/15g) (for super speed. you must have room and time to charge/sidestep sirens/crawlers - or take them out with 9mm - and at later waves you need to know how to one-attack-decap fps).
On tight maps with corridors and corners such as office and to some extent bioticslab I'd say chainsaw+katana work best for me. On open maps where you can sidestep, kite and time your attacks such as farm and to some extent london I'd say katana+bullpup+handcannon works best for me.
Eventually, when I practiced to one-attack-decap fps with the katana, I will probably switch to Katana+Crossbow or Katana-and-nothing-else as my standard Berserker outfit on all maps.
... I think most people currently using it are just Katana fans.
I think most berserkers use Katana because hit n run is a really fun play style. Before patch there were a lot of berserker and medics that used Fire Axe for the same reason. They are also fed up with the constant chainsaw sound and the fact you move so slow that your team members kill your targets before you reach them. The Katana has weaknesses, so do chainsaw, but Katana is fast, powerful and fun to play with - chainsaw is "only" powerful.