I use my full suspension mountain bike when riding with my wife on her Townie. It's a lot easier to go slower and stay with her pace than if I used my BMC Racemaster road bike.

You also mentioned 40-80 mile charity rides. For rides like those, I'd use either my BMC or drop-bar Bianchi commuter (basically a cyclocross bike) for the variety of hand positions available on the drop bars.

You've got the slow bike already, so I'd say to seriously consider the cyclocross bike (or a road bike, for that matter) to add to your stable. Keep the Hardrock and get rid of the 730 (IMO) so you don't take up any more space at home, if necessary.