I am another who has multiple packs.

My current rotation is a Gossamer G4 for general three-season use. This is my "go-to" pack for for anything longer than a weekend. For winter use I take the Golite Pinnacle, not really because it has a larger capacity, but because it is just more durable than the G4 and I know that my snowshoes won't chew the pack up.

For shorter trips I take either my Golite Jam (weekenders) or MLD's Burn (overnighters, day hikes, day fishing trips, summits etc). I am currently trying to fade out the Jam in favour of the Burn, even on multi-day trips.

For shorter hikes, less than seven miles or so, then I will just take a hydration waistpack since all I would really need to carry is a litre or two of water. It makes no senese to bring an actual pack for this purpose.

I guess I just like having the right tool for the job. Personally, I couldn'y envision taking my 72L Pinnacle on a day hike-- all that loose pack, sagging in the most inconvient way etc.