I used to have a shed which I used on my daily commute to work alongside my BMW 330ci M sport. I ended up exactly how Marzman explained, the BMW became too 'nice' to drive and I ended up hardly using it! It came to a point where I only used it to go for a 'round the block' drive solely for pleasure. Due to the lack of spare time, I never had the chance to take it out!
I also fell into the trap of not treating the shed an actual 'shed'. I started off with an old L reg starlet which I got bored with. Then I upgraded to a BMW 318Ti compact which felt under-powered. Then I bought a Toyota MR2 GT, now that completely defeats the object of having a 'shed' as a second car.
I then realised it just was not worth having a shed at all as I was clearly worse off and decided to drive the 330ci everywhere. The savings of not running a shed on the side would cover the expenses of fuel, depreciation, wear and tear etc.
I decided to sell the MR2 (which I made a nice profit on) and just had one car. To reward myself for seeing sense, a few months later I got rid of the 330ci and treated myself to the Zed
I've learnt that having a shed makes you more and more insecure to take the Zed out (or your actual car) as the shed will make you feel the true value of your 'proper' car. You'll ask yourself, why shall I risk taking the car out when I have a shed I could use and risk?