I would find it useful if Edit > Show All Masters also worked in Text mode–particularly with selections longer than one letter. I.e., if I go to text mode, type a word, highlight it (by shift-clicking the cursor at the beginning of the word), and select Show All Masters, it would be nice if Glyphs pasted in the whole word in all masters.
(If this can’t or won’t be arranged, I’d suggest graying out the Show All Masters menu item when the present mode will prevent it from doing anything.)