it doesnt 'hurt' anything per say. but since 95% of the AOA ores are at y9 or less having bedrock that takes up y2 thru y5 tends to reduce the already rare ores and drastically increase the frustration level. in my personal exp i was able to find 12 saphire ores in 3 chunks. and when others tell me they found 'alot' and alot = 7???
Flat bedrock does slightly inctease the ore spawning amount vs normal bedrock, yes. Still perfer normal bedrock.
the world is generated with normal bedrock we are not going to change it soon. only if we reset the servers we might change it