It's an interesting question. One I've been considering myself recently. I'm away at the moment on a road trip round France and have brought along an 8x30 porro and a 8x20 leica trinovid br to try and see if I could be content (i.e not optically frustrated!) with just an 8x20.
I've been genuinely surprised by the 8x20 in a lot of circumstances. I've used it a lot over the last few months since I got it back from a full service - that in itself says a lot as I don't typically like pocket (20/25mm lenses) binoculars.
The handling has taken a bit of familiarizing but I'm quite happy with the stability of them now. Eye positioning is fiddly - there is no denying that, I like a 6mm+ exit pupil for a reason! They are very useable though with careful ipd adjustments and positioning of my hands to lift them slightly away from my eye sockets.
Outright resolution is not bad, on a heat hazy day today at the mouth of the river somme they were about as good as a full size set, in more settled conditions they are clearly not as sharp and accomplished at resolving detail as even an 8x32.
The real problems though for me at least were highlighted when we walked into a church. I know this is bird forum but me and the family like to go into any church that has it's door open on our trips away and I always enjoy getting closer to the architectural details, paintings, organ pipes and stained glass, whatever, with my binoculars. There's often something special about the craftsmanship when it's been done in the name of faith.
The 8x20's were a real let down though. Visibly dimmer than naked eye and as a result scarcely revealing more detail, a quick check on my pupils revealed why, I was loosing about half there light gathering potential with the bins misery 2.5mm exit pupil. This has also been born out on cloudy days and dawn or dusk use - forget any engaging viewing of the night sky too.
So I suppose that's the advice, pockets are great on a sunny day, 8x20 or 10x25 have the same brightness so will be equally accomplished on those occasions - just pick your favourite mag but if you want a smaller second set that does most things well and can show you the brush strokes of a fresco, reach for a compact 30 or 32.....