The ability to sign posts is useful outside of talk name spaces. For instance http://en.wikipedia.org/wiki/Wikipedia:VisualEditor/Feedback is in project space. While that might one day be in flow there are many other project space items where its useful to be able to sign. For instance lists of project members. Some uses have guest books in the User: space which can be signed.
You can't even enter them manually due to bug 49820.
A button to insert this would be good, possibly done at the same time as bug 38029 which would insert symbols.
This was discussed at http://en.wikipedia.org/wiki/Wikipedia:VisualEditor/Feedback#Let_me_sign.21