This is a valid technique if you do cold extraction (e.g. cold brew). I often reuse coffee grounds and mix them with fresh coffee when cold brewing with no part

This is a valid technique if you do cold extraction (e.g. cold brew). I often reuse coffee grounds and mix them with fresh coffee when cold brewing with no particular downsides. Cold brewed coffee also lasts much longer before going into "tastes like old coffee"-territory, you can have it in the fridge for weeks while hot brewed coffee can taste bad after a few hours already. It is said that the acids in the bean play a role in this, and those acids are not extracted during cold brewing. There is people experimenting with ultrasonic extraction processes as well.