Knock Off, in Gen 5, is base 20 power and does not get any boost when the item is removed, as you already said. Seeing this, it would not be wise to use it on any offensive Pokemon that may use it as a STAB move or coverage move, for example Weavile in Gen 7 and 8. Defensive Pokemon's don't really mind the less power of Knock Off, removing the item maybe it's sole purpose. Other Pokemon's, such as Suicide Leads, may also utilize Knock Off in some way or the other, as their main role is to set hazards / not let hazards be set up, and then die. Removing items can be important, especially Leftovers from a few defensive Pokemons. Removing Life Orb's and Choice Items is also a relief during the latter part of the match. Let's move on to the Pokemons who can use Knock Off.
- Azelf: Great suicide lead, Stealth Rocks / Explosion / Knock Off / Taunt seems good.
- Clefable: Defensive Clefable's can sack some other move to keep Knock Off, but this is conditional. Clefable usually likes to keep other moves, like Stealth Rock, or Encore.
- Ferrothorn: Same reason as Clefable, it can utilize Knock Off at least to some extent.
- Whimsicott: It can also keep Knock Off, but removal of some other move such as Thunder Wave may be a problem
These are just some of the few Pokemon's who can make use of Knock Off in Gen 5 OU, as base 20 power is not screaming a sweeper. Other Pokemon's who don't care about the move's les power can also use it.
Hope it helped :3