Oh, you could fit spare tyres, mechs, handlebars and almost a whole bike in a pack.. if you wear one. I don't. All my stuff goes on the bike; bottle cage and seatpack, etc. It's a trail bike, why do you need a pack for that? It doesn't tend to be an epic wilderness adventure, but rather a few hours singletrack in british countryside. All I need is a 500-1 liter bottle & a few spares (possibly a layer too)- not a 25 liter bag.
So for me, it's essential. Many would agree, hence their existence. Even on the 'enduro' Airdrop Edit people were outraged* at the lack of mounts on the press release images.
Yes, I can't that geometry and component choice are arguably higher up the pecking order of being essential. I never said they weren't.
Anyway, that's just my opinion. Wouldn't it be boring if we were all the same..
*slight keyboard warriorism