Yeah agreed, Eos does good damage. (Some very quick and dirty fsimming makes me skeptical that you can be confident that "You will do better with +9 eveningstar." I'm sure it's true for some characters and some enemies though.) It seems to outdamage Leech by a solid margin at comparable skill levels and against electrocutable enemies (but again, quick and dirty fsimming with normalish stats, not trying several combinations of str and slay rings etc
).
As mentioned berserk is often a great buff, and this does make *rage less bad. I'm not sure how much ability to go berserk is a point in Leech's favor over another weapon, though---carrying Leech effectively lets you berserk at will with other weapons too, since you can swap to Leech, evoke berserk, swap back, and pound away. If you don't mind the annoyance of playing around *rage, it's not a huge downside for the reasons people have mentioned, but I think on balance it's most of the time worse than just having a swappable +rage item in inventory.
So it seems to me that, assuming you're willing to pay the annoyance cost of *rage, the next big question is how much you value vamping on every hit vs more damage.