MattKing and Newt_on_Swings gave some good examples where consistency wasn't happening for what should have been routine darkroom work.
2F / 2F, great advice to throw in an ND filter.
I like my old Omega A.R.T. and try to keep it at 32 seconds. My third f/stop times from there are easy to remember 7-5-4-3 and I just count that many clicks while burning and dodging.
When I start out, I turn on my lamp and let it warm up a bit. I put a meter similar to Nicholas' under the brightest (shadow) spot and change my f/stop on the enlarging lens until it "nulls" at the point where my time should be 32 seconds.
To burn, I'll set the A.R.T. to 40 or 50 which is the next two third-stops up from there (nominal print time is still 32).
Even still, I sometimes have print-to-print variations that I blame on variations in brightness of the old fluorescent ring light source. I think it is funny that the Omega instruction booklet claims at great length how their large ring fluorescent tube is instant-on and provides more stable light output than competitor's grid lamps.
So the value of a timer is consistency - which will reduce frustration - which is priceless.
Assuming you have one of these light sources... I also assume your light is stable by one of these methods.
-With a grid lamp, I'd want to either have a compensating timer -or- a "Horowitz" stabilizer.
-With an incandescent lamp, I'd want to have a voltage regulator.
-A colorhead probably has a matching regulator.
-With my setup, if you believe the literature it's OK as-is.
But with my setup I'd want a compensating timer. For now I am settling on my old "null" meter and pledging to check before each print.
I live with the variation by making prints in "sets of three" - one for me, one for others, and one for mistake. One of the three can be wrecked and I still call the day a success.