Each to their own.
I have the tail pack & it's on there all the time. It's so handy to carry stuff, I've got a puncture repair kit in there, a compressor / battery jump starter, some tools, cloths, visor cleaner. That lot always stays in there, plus when I'm using the bike, my wallet, cell phone case & 750ml water bottle are also in there. To say you can't get much in there is incorrect (imo) and in the side pockets I keep the fob & spare ear plugs in 1 & can't recall what I put in the other side pocket. Edit: I've got a full set of spare fuses & fob battery with sml allen key.
The last good thing about this tail pack is it acts as a back support, ride with out it then ride with it, it's amazing the subtle difference it makes - well it does for me anyway, yep, I'm a fan of this wee bag, not so much the tank bag, got that too but never use it.