It's not illegal, but it's stupid except in dire need.

If your partner goes into cardiac arrest while you're driving, what are you going to do? Stop to apply CPR and while simultaneously calling 911? Start driving faster, running red lights, and hope you both survive the trip?

Depends is your partner driving you to the ER in an emergency?

Or is your partner offering emergency services to the public in exchange for money? If so is your partner properly licensed and registered to provide those services? Is your partner insured to provide those services?

That's an entirely different point. If you're out in the middle of nowhere or even the middle of downtown SF, and you cut your foot off, you can decide to do whatever you want about it. You could call an ambulance, have your wife drive you, or just lay there and bleed because you don't want to be an inconvenience. Freedom of choice, all legal options.

If you went to a licensed medical professional with your foot cut off, they're not allowed to call you a cab. They have to call an ambulance. You can tell them, to hell with your fancy EMT's and expensive rides, I'll just have my wife take me over to the ER to get this foot sewn back on, but they have to call an ambulance if you need transport. This is a liability and safety issue. It's also a kickback issue.

TL;dr- No, it shouldn't, and nobody wants to make that illegal.