In using my watchlist, and my (and others) contributions lists (among other things), I think that adding 2 lines to the "Namespace" filter choices would be most helpful:
All pages (no talk)
All talk pages (only)
(If anyone can think of a better way to phrase them, that would be welcome : )
There are times when it would be helpful to just view an editor's talk page edits, and sometimes, to just view an editor's edits that aren't talk edits.
There are something like 9 different namespaces (and 9 talk namespaces). When looking at my watchlist (for example) it would be ''much'' easier to look at all talk namespaces to see if any talk page discussions have continued. It also helps when going through someone's contributions list to see what what discussions they are or have been involved in. (Which is '''''very''''' useful when trying to track a multi talk page discussion. There have been discussions which have wound through template talk, category talk, image talk, several Wikipedia talk pages, and many user talk pages. And that's just a single incidence.)
On the converse, it would be useful to see what edits a user may have made, without the distraction of a myriad of talk space edits. Tracking vandalism is just one application. It also would help when trying to undo a slew of POV edits from a user. And for my watchlist, it would make it much easier to check watched pages (such as articles, categories, templates, Wikipedia policy pages, etc), without the distraction of a myriad of talk page discussions.
One typically only views so many edit instances on a single page. This would also aid in page by page viewing of edits (such as for the applications above, for example). Etc.
In searching through existing bugs, the following two seem to have similar themes:
11036 Search: add "all namespaces" shortcut
11265 multiple-namespace filtering
Though after looking at them, I don't know if having checkboxes is better "spacewise" (the drop down box seems easier to use/takes up less space/etc), though checkboxes seem to work fine for searches. (And having these two talk spaces options added to searches would be nice as well : )