When would a nexus be useful or fun in a custom map?
I really do wish you would use spoilers when discuss something that is related to the end of CW2 and may contain spoilers :-[. Although I would see this as an opportunity (if used right) as another custom boss battle.
A nexus is very useful if you want a boss battle, but also if you want to remove the blocks around the nexus to make room for the boss battle (used this in day 26). If you want a good boss battle without a DB, use normal emitters and gateways.