I rode in ERG mode for the same reasons as most:
1) It kept me locked in at an exact power, especially helpful on harder intervals where my tendency was to back off near the end
2) It allowed me to just space out (listen to an audiobook/postcast/whatever) and all I had to do was turn the cranks
The arguments against ERG mode:
1) Even when riding at the same power at the same RPM, the muscles you engage and the timing of that muscle engagement are different depending on what gear you are in. If you live in Miami and are training for IM FL maybe that's not so much of a problem if you do ERG mode in the same gear you'd normally ride, but for those of us who live in places with hills and/or race on more varied terrain, this is a major problem. I used to ride ERG mode in the small cog in the middle of my cassette (for no specific reason, but this simulates "hill" gearing), but now that I'm out of ERG mode and spending more time in the big ring, I'm finding I'm much more able to turn a big gear on longer stretches of flat. Wahoo explains this here: https://blog.wahoofitness.com/...ar-selection-matter/
2) While it sometimes sucks, you really should be focusing on your riding form, pedal stroke, body position, etc rather than watching Netflix. I recently when back and compared the pedal dynamics from my dual sided Pioneer PM for workouts where I was very focused on the task vs spacing out watching Breaking Bad re-runs, and the difference was pretty eye opening. I'm a busy guy, and out of ERG mode it's clear I'm getting more bang for my buck time wise.
3) Now that I'm riding out of ERG mode (and paying more attention to my body during workouts), I'm developing a much better awareness of the correlation between perceived exertion and power. I find when I'm riding outside, when I glance down at my PM, my estimated power is pretty darn close to actual. This helped me in my most recent race, because I was able to hold my power target much more naturally (without glancing at my head unit every 30 seconds) and instead focused on terrain management, etc. I didn't realize how much I was relying on ERG mode to keep me in my power zone until I started training without it.
I know many have had success with ERG mode, but just though I'd share my experience. This is just N=1 of a middling cyclist who really struggled to make gains on the bike over the last year but since "quitting" ERG mode in May my improvements on the bike have been very satisfying. I'm sure there are other much more skilled athletes and coaches who have a deeper perspective here, and perhaps they can weigh in.
(ps edit) if you are a 30-34 year old male participating in Santa Rosa 70.3, Santa Cruz 70.3, or contemplating 2020 IM Mt. Tremblant, please disregard this post and continue to exclusively train in ERG mode).