Did you see the recent spider-man movie? He fights an evil scientist-turned-lizard and he has the ability to regrow limbs at an astonishing rate. However, he suffers no consequences for doing so.
This is, of course, preposterous.
Consider that when you regrow a limb the mass of said limb has to come from somewhere. What are you sacrificing in order to regenerate that tissue? Muscle mass? Fat deposits? Secondary organs?
For a limb to grow that quickly your body would probably go into some kind of shock, as it is being deprived of all of its energy deposits in only a few seconds. It would be incredibly traumatic.
Even growing it over time would take a toll on your system. You'd probably be very tired, eat a lot more, not be able to maintain/gain, or even lose, muscle mass, etc.
Furthermore, how powerful would that limb be? Would it have the musculature of your existing arms/leg? Probably not. It would be incredibly weak, and vulnerable, and in need of therapy to use as your uninjured one.
if your metabolism is capable of accomplishing this feat, how else might your organism be impacted?
How would your immune system react to viruses, or bacteria? Would you ever catch a cold, or get an infection?
Would you need to build up massive deposits of fat "just in case" you suffer an injury? Could your body trying to grow a lost limb potentially exhaust you to the point of entering a coma, or even killing you, if you don't have the resources in your body to provide the "building blocks" for said limb?