I think that just having one bionic leg should not be as simple as getting half the benefit, because having two balanced legs is more than the sum of it's parts.
I think I would downgrade it one step and then cut the benefits (if any) in half. This might make the replacement of your other "perfectly fine" limb more of a necessity due to the limitations of your bionics than a choice of an upgrade, since a Common-Quality bionic limb would become a half-bonus Poor-Quality limb instead, due to issues such as weight distribution, relative endurance, etc.
Possible in-universe reasons for not wanting to cut off the second leg:
1. is horrified by the idea of replacing perfectly good flesh with technology without exceptional reason.
2. is anti-tech (probably not for this character in fairness unless they're quite mad)
3. understands that technology has faults and can fail at inopportune moments
4. doesn't want to spend even more time maintaining gear (which now includes parts of his own body)
All of these are perfectly valid reasons. I mean, c'mon. Most people wouldn't cut off a perfectly healthy leg just to replace it with a "better" upgrade, or they would already have done so with both their perfectly good legs long before whatever accident caused their predicament.