In my experience, a prototype is never just a prototype. You crank out a prototype in an hour to demonstrate something. Then somebody says "add this and then we'll show it to the boss.". Your boss says "that's cool, add this and this and this and then we'll demo it to a potential customer". You do so, then play test the demo a few times, uncovering scenarios that need hardening. You demo to the customer, and they ask a few questions meaning that you add a few more features to the prototype for the next demo. Rinse and repeat.

After a few successful demos you're given an tight deadline to "finish things", so you have no choice but to polish up the prototype and ship it.

The moral: code that was written to be thrown away often isn't. You can still write code quickly, and take shortcuts (such as skipping tests), but recognize that you'll probably be working on that code for a very long time and may regret your decisions later.