Advantages to keeping old road bike after getting a new one? (medium-length)

[Pedal Wench]

Not to mention the obvious advantage of keeping the old one while buying the new one: 2 bikes is always better than 1, correct? And 3 better than 2, 4 better than 3... and so on.

10 better than 9, 11 better... well, you know the drill.

Bike rack bike when doing a ride that someone else is driving too, might rain bike, might be dusty bike, climbing bike, heavier training bike. Or, can't bear the thought that something might happen to it bike. That's the one that gets me.