I think riding should be about fun as well as getting to where you're going. These days I've got a "fleet" of 6 that see regular commuting duties and there's no real primary bike since I like them all for different reasons. And the fleet is flexible with bikes coming and going on roughly an annual basis.

But I remember back to my early days of innocence when I first made the leap to double the fleet to two.....

Depending on what you have now and how adventurous you are I'd strongly suggest getting a bike in a completely other style just for the fun of the differences. If you've got a mountain bike then get a touring or cyclocross bike. If you've already got a drop bar bike then get an older cross country mountain bike with a nicer frame and set it up for a more casual quick errand bike. Or perhaps do a single speed conversion of some nice quality old warhorse. Or maybe if you don't get much rain down your way a full on road racing bike. Although if you have to lock it to racks then you may want to find an older less "shiney" option so it's not so tempting.

All in all it depends on your situation and how you like to ride but it's nice to have something different from what you have now.

And it doesn't need to break the bank either. Years back I found an older basic CrMo frame for $14, some used wheels and a few other bits and this is the result that I'm still riding and enjoying today. Recently I upgraded to the mustache bars and some Tektro long reach dual pivot brakes and it's like a whole new bike all over again.... All for around $200 total investment.