I have played two warriors with Soul, and I wouldn't say it's a "must have," but I
would say that over those couple hundred hours I was amazed how many times
I'd finish a battle or a raid and have less than 200hp left.
Each time I found myself in that situation I was super-glad that I had made that
choice. It's not an exciting choice, by any means, but IMO it can make the difference
between living and dying often enough to be seriously considered by any warrior.
For the record, they were:
Dastmachi Dagger/Whip Soul/None Village Defender
Landser Sword/Flail Soul/Fires Village Berserker