Suggestions for SK

Is acumen even valuable for the SK?

This should probably be in "Player questions". The buff is certainly valueable, for an SA on a backup weapon I would recommend Magic Power instead, since with the SK's low MAtk it actually pays off.

I knew an SK who had a Som with Magicpower so he could switch between his SH and SK sub and save on money

58 lvl sk has about 170 m.atk, the SoWD with magic power SA would boost this number by about 90.This would increase the chance of sucsessfull debuffs greatly. the increased c.speed that acumen gives isint so important like increased m.atk. Besides it would be even cheaper and more effective to PvE with sowd:
You would get more hp from mobs with lifeleech+sowd+bspsC then with damascus+focus+bspsB

So no, the acumen is not important for SK, it would only increase the speed of your pve, you will save some time while casting debuffs etc.

before pvp you should also ask your friends for empower instead of acumen, or ask for both :)