vIghro' legh vay' 

someone sees the cat 

vIghro' leghlu' 

someone unspecified sees the cat 

leghwI' 

someone who sees 

So, why couldn't the {leghlu'wI'} mean "someone unspecified who sees" ?

-wI' turns the verb into a noun that represents the subject of the verb. In leghlu', there is explicitly no subject, so there is nothing to turn into a noun.

Or to put it another way: what's the difference between someone who sees and someone unspecified who sees?

-- 
SuStel
http://trimboli.name