One bike is fine, as long as you have extra wheels. I think the most common failure is when you roll your bike to the door for the morning commute and notice that one of the tires is flat. A quick wheel change, and Bob's your uncle!
I would only use a beater if the ride wasn't too long, and there was no secure inside bike parking. If the bike has to be left outside, locked to a typical rack, then a beater is the only way to go. And beater wheels are usually non-standard (what fits on one won't necessarily work on another beater). So a second beater for those flat tire mornings would work. So if this is your situation, I'd keep both beaters.
I happen to work where there is a locked bike room, and there's a shower facility attached to the office. The office is also 16 km from home, and my route to work is usually 19 or 20 km, so I always ride my good bike. The commute is usually the best part of the day, so I want something more than just transportation.
Luis