I think full name mention is better in many cases and near essential in some scenarios.
- Full names read better in paragraph oriented conversation as opposed to Twitter style 140 character cryptic text.
- Names rather than IDs are more friendly for not technical people.
- Languages such a Arabic, Hebrew, Urdu etc. in which text flows right-to-left (RTL) do not look good with mixed in LTR mentions in ASCII/Latin characters; full names in the native language would fit more naturally.
- Many non-Latin languages require changing keyboard to English just to mention a user then switch back to the native language is a pain; Facebook and XenForo have solved it more elegantly.