What is the need for a thumbs down icon?

[Maverick]

It seems to have no effect.

[Sazzouu]

Ordering items with the same amount of Thumbs Ups.

[Ogami]

It has, its just invisible. Dislikes are not displayed but still counted.
So lets say a review has 100 thumbs up and 30 thumbs down.
You only see the thumbs up. Another review has 110 thumbs up but 90 thumbs down.

The first review will be displayed over the second one as more useful because it has less thumbs down.

The reason the count of thumbs downs was made invisible is so trolls cant easily mass downvote user made content. Since you cant see the downvotes organized mass downvotes are much harder since you cant really see a direct result of such a action.

[JVC]

I does. Try looking at the announcement for Europa Universalis 4, Leviathan. On launch day it showed zero upvotes and 700 downvotes because the huge majority of no-votes cancelled out the yes votes.

[UCEY]

They're counted, but not necessarily displayed. Take addons for example. Out of 5 stars, one addon gets 3 stars. What does this mean? For example, 5 people voted on the addon, 3 were thumbs up and 2 were thumbs down.

[DaBlanka]

[Tito Shivan]

Originally posted by Ogami:

It has, its just invisible. Dislikes are not displayed but still counted.

With the exception of users which are found to be spamming the downvotes (As bots do)

That's mostly why they're not displayed, that way a bot can keep spamming to his heart content with zero effect on the system. While if downvotes were public they could instantly see if their votes are counted or not (and adjust behaviour to not trigger the alarms)