Sandman - personally I don't agree with any excuses for poor preparation and insufficient practise...

[ I hold my hand up to occasionally not learning new songs until the last minute before a gig,
and then being too stressed out to remember them...
In a band I could just turn my instrument volume down and mime ]

But am happy to make allowances for individuals with memory impairing health conditions,
using tablets and phones with lyric reminders..
providing they can still sing and play instruments to an acceptable level of proficiency...