A timer is just like any other tool.
It can be helpful in gauging general progress (or lack thereof), and to a lesser extent lap to lap consistency (being that there's so much variability on track during any given session - was I in traffic in lap 2, 3 and 4? was there a bike between me and the beacon?).
Problem is, people misuse the tool and blame it for their own shortcomings, sorta like blaming the hammer when you hit your own thumb - it's not the timer's fault if you stare at it obsessively while you're riding, nor is it the timers fault if you have a miserable day because you think you're riding faster than your timer reports (or god forbid, someone doesn't put a beacon out...).
By their very nature, GPS timers seem better, simply by the sheer volume and general increase in quality of data available to the rider, let alone the lack of reliance upon said beacon and the accouterments and conditions required to keep the beacon working and a signal picked up by the on-board unit, but at this point, you're on the front-end of adoption. Being that, GPS timers cost is a few times more than an old-fashioned lap timer that we all used up until last year... Give it a few years, and you'll probably never see an IR beacon on pit wall at the track, and we'll all be carting around laptops to dissect our riding, session by session.
If you want a timer, get a timer. I like mine, but half the time, I don't even put it on the bike anymore.