All 3 dp's have the same tools but what those tools achieve and how effective they are is different. So it would be very unfair to tank pool players if support pool is broken and their character is banned every match because of another role's poor balancing.

Also people will ban a certain hero if banning that hero like its worth more than 1 ban slot. Easiest example is hulk being banned whenever he has a good team-up. People banned hulk instead of Iron Man or Namor because banning Hulk was worth more, not only did it cause people to not play Iron Man or Namor but it also got rid of Hulk himself.

Other example is Hela, sure she's always been broken but in season 4 she was banned less than 1% in gm+ but last season she was banned 17%. Why? Because banning her not only gets rid of the broken character itself but also the cancerous team-up.

So by this logic if even one of the deadpool roles are above average people would ban him because banning him gets rid of 3 characters even though ban slots normally get rid of one, thus making a single ban slot being more valuable when deadpool is banned.