If fully charging and discharging a battery gets you 300 cycles till battery failure, and 50% discharge is getting you 600 cycles, is there any advantage to not deep cycling lithium batteries?
Yes, most definitely. All the electric autos purposely limit the depth of charge/discharge to greatly improve the life of the battery. Charging a lithium cell to only 4.1V and only discharging it 80% can extend the life of the cell over 6 times one that is fully cycled (like from 500 full cycles to 3000 partial cycles). Also, for longest life, keep the cell charge/discharge rates under the cells “C” rating.