We are all aware of the achievements. You know the badges you get for edits somewhat? It used to be in the wiki features. Now, you have to bribe-I mean ask Staff to enable them. So why should you or should you not use badges?
- Editor morale - Hence the name, achievements can give one a sense of achievement and a badge can show dedication hence improving the editor's morale.
- Improved community - If there's a reward, some may want to start editing and it might even be able to grow your wiki.
- Competition - As opposed to competing for who has the highest editcount, hence causing cheats by userpages, you can have competition for who's on the leaderboard and usually badges are given for mainspace edits.
- Cheating - Cheating for badges can be an issue on wikis and it has happened before. This is when one makes minuscule edits on a page multiple times when it can all be done in just one edit.
- Leaderboard may not be representative - The leaderboard has the top 19 users but the top 19 may not actually be representative of the wiki's active users and may be of inactive ones which seems unfair.
- Edits before enabling - Edits before enabling achievements don't count so you can have like 10,000 edits but none of that gets rewarded which may lower people's morale and self-confidence.
- Encouragement of edits that are not needed - This is in good faith but some may edit when it's not needed like adding categories unnecessary to get the category badges. (Credit to DEmersonJMFM)
- Does not factor in quality - Quality of edits are better than how many edits you have to many but the achievements do not factor that in.
There may be more but that's all I thought of for now.