Confession: I am also a multi-accounter myself. I have 2 Instinct accounts (L37, L34) and 1 Mystic account (L24). A few reasons:
1: I started to level up the 2nd Instinct account in July 2017, when legendary raids started. Back then I didn't bother with gyms, but I decided I needed another Instinct account to survive in my Mystic-dominated area.
2: Multi-accounting was already very rampant here back in July 2017, and I didn't even know it was cheating.
3: I stopped playing with the alt account for a while, and I found that I couldn't stay in gyms for long with only 1 account, as I would often hit the 10 berry cap, and other teammates didn't even bother to feed. So, the alt account was the only way for me to survive, even if it was cheating.
Anyway, the only single-account legit players left in my area are all Mystic.
4: I use the Mystic account to shave off only myself, teammates who have already collected full 50 coins for the day, or teammates who ask me to shave them off at a certain timing, and let me slot my mon.
5: I use only 1 phone. But sometimes, I borrow my friend's (doesn't play pogo) phone to duo Ttar and Absol, and as backup when attempting hard T3 soloes like Alakazam.
Anyway, my 2nd account is used mostly for defensive purposes, and therefore has mostly defensive mons. It has a CP 3022 96% ZH/DG Blissey, which I spent over 100 rare candy evolving and powering up from a low level Chansey. (It was the same Chansey spawn which I got my main account's CP 3160 96% Blissey from).
Other than that it's got a C/IB Slowbro, 2 C/DG Gardevoirs, 1 each weather boosted Snorlax and Chansey (TMed to ZH/HS and ZH/DG respectively), and a C/DP Machamp/Hariyama army for backup gym offense and duoing T4s with my main acc. The only legendary I've powered up (and maxed) on this account is a 100% Kyogre.
Yeah, downvote me all you want. I'm fine with it.