@Thisfox Let's say you decided that you want a glass maker (and you didn't have one before). It is nice to be able to look at your dwarfs to see which one has glass making skills. If you use 'u', you have highlight the dwarf, press 'v', keep track of who it was, press 'esc', press 'u', highlight the next dwarf, press 'v', remember if the other guy had higher skill, so on and so forth. It's complicated.
What I do is put all of my dwarfs in squads (whether or not they ever do any military service). Then I direct the squad to an empty room and wait for them to show up. At that point you can use 'v' from the main window and position the cursor over each dwarf in the room. No juggling at all. You can compare dwarfs easily. No problems. Then when you want to change their roles, it's the same thing. Just move the cursor over each dwarf and change their roles. You can move as many squads as you want into the room. You can even select individual dwarfs from the squad menu.
Even if someone is not present in the room, they are either sleeping, drinking or praying, so they are easy to find. DF puts active squads at the end of the 'u' list, so you can work your way up from the bottom looking for dwarfs that didn't get stationed for some reason.
Usually I work with a 2 squad fortress. In other words, all of the essential roles in the fortress are split up between the 20 dwarfs in 2 squads. Then I have redundancy by doing the same thing in a separate 2 squads. To be honest, there is almost nothing that can't be done with 20 dwarfs, let alone 40, so after that I just get a bit creative, or swap people through military service or whatever. It's pretty flexible. I usually also keep notes in a separate application, but it depends on how efficient I want to be.