It shouldn't be an either/or thing.
There are drills that are best shot without time pressure. Improving marksmanship fundamentals would be a great example. Even if you don't want to do NRA-style slow fire you still shouldn't go faster than you feel you're getting your
best visual reference and your
best trigger manipulation.
There are drills that are best shot with some time pressure. For example, speed-oriented drills.
I know, I'm genius. Even with speed drills, though, you don't necessarily need a timer for every run. Work on pushing yourself to the point where you're almost but not completely in control of what's happening. You're pushing yourself but not running blind.
What you need a timer for the most is
measuring speed performance. It's just a complicated stopwatch. So when you want to know how fast you're doing something, you use the timer.
Some people like to use a timer for every single rep. Personally, I don't find that helpful.
I find
myself getting too focussed on the time and wanting to scrape a few hundredths off each run and not paying enough attention to performing techniques properly.
I end up getting a fraction of a second faster but compromising things that are more important to me than raw speed. A lot of those things don't have much to do with USPSA/IPSC/IDPA, though, and might not matter as much to you.
I like using timers on PAR (or a target system that lets you do essentially the same thing) for things like
floating PAR drills. It still has the possibility of tricking me into getting sloppy with my technique priorities, but as soon as something goes wrong it resets the clock and essentially "tells" you to slow down a bit.