In our company we are using the following scenario:
Customer users don't have Browse Users global permission (we dont want they have possibility to see other customer usernames) but they have View Voters and Watchers permission and Manage Watchers.

As a result of this is that they can add watchers to the issues but they have to know exact usernames. They can do it in Issue View but they can't do it in "Detail View". This is caused probably by request

/rest/api/1.0/users/picker?showAvatar=true&query=s&_=1395241798570

which is sent some onKey event. This request is not sent in Issue View.
For me would be the best if you completely get rid of this additional request in Detail View.