Well, no matter who you are, we all agree that a tool that shows every single skill and its progression on the one screen is unbelievably useful if you need someone for a specific job and couldn't be bothered looking through 80-200 dwarves individually. You can just close it straight away after 2 changes if you're worried about performance. To say that this is more time consuming than individually monitoring dwarves and their skill progression... well...
This probably doesn't apply to all players, but DT is more time consuming
because it lets you monitor and compare every single skill and its progression. Dealing with that much information makes me much more unsure about labor assignments - I need someone for a job, but the guy who's most skilled in it usually has some other great skill that I don't need right now but might be useful later, or he has too much combat skill to be a whole-time craftsdwarf, or whatever; something always comes up that doesn't make the dwarf quite perfect for the job. Going through migrant waves was just such a chore, and after committing the job changes I was never happy about it.
So I stopped using Therapist, just to see what it's like without it.
And as it turns out, my playstyle changed quite a bit. If you have 50 dwarfs, a job that needs to be finished and no way of finding the dwarf with the highest suitable skill level, what do you do? I open up the units screen, pick any idler and give them the job. If they need to do it often, they'll become professional, if it's more of a once-every-season kind of job they'll idle again and pick up more jobs.
I know that I won't pick the optimal dwarf for the job like this, but the dwarf will become great at it over time, regardless of their initial skill level. After a few years, the result is the same regardless of which dwarf I picked - jobs get done, dwarfs get skilled. And I can save up some brain cycles for more important problems.
To summarize, I like to play without Dwarf Therapist because it limits my obsessive micro-managing.