Don't have a wife, but I've let my best friend drive it. She knows how to drive stick, but the M3 is completely different than what she normally drives.
Why do I let her? She's my best friend. If it makes her day just to drive my car 10-15mins, then great. I believe in Karma. Plus, she knows the consequences of messing up my car haha.
Current: 2008 BMW M3 Sparkling Graphite/Bamboo Beige/6MT. 1999 BMW M3 Techno Violet/Dove/5MT. On the list: BMW M2 Long Beach Blue/Black/6MT