I had this very same discussion with my mate yesterday and was as well wondering why he wasn't played as melee support like Tide or ES. He has pretty much the same stat growth than the 2 others, same move speed as ES and even an increased night vision 1200 when the 2 others have 800.
I wanted to see if we could consider picking him when tide is banned or picked by opponent.
Moreover he is the first pub stomper (1st winner win over 60% win) and the rest of the top 5 heroes are either perma ban (lycan) or often pick (lich/ursa/tide)
He is the only one that is just never picked by pro. (4 times over 1500 games played in one year)
I'm not saying he should be perma pick/ban but I'm really wondering why he isn't in at least 5-10% games like batrider (which is terribad nowadays)
The only drawback I see compared to tide/es is that the range of Leech Seed is way too small, but do it makes him unplayable in CW ? I'm really not sure and I'll probably play him with the exact same role as Tide/ES because I wanna know.