The existence of an underpowered option does not mean that more options don't generate more power. Because options you don't choose don't make you weaker.
More options, unless they are all really bad, means you can sift through those options to find ones that make your build stronger.
The more powerful the thing being swapped around, the bigger the impact; being able to choose the height or weight of your PC is an option, but the impact is small.
Swapping one strong spell for another has a large impact. Even if they are both awesome things, the ability to pick one of X awesome things is stronger than having any one picked for you.
Both PWOT and Shadowblade are awesome spells. Which is better is going to be very build, party and situation dependent; but very often, it will be build and party.
Shadowblade isn't all that strong on most wizards, because most wizards really don't want to enter melee. Shadowblade is quite strong on someone with many attacks and the ability to survive in melee.
PWOT is quite strong if you are the only person in your party with it, and you and/or your party chooses to stealth, and exploration pillar is important at your table, and your DM runs stealth as useful. If any of those aren't true, it isn't nearly as strong.
Getting Shadowblade on a high-tap-count melee-capable character with high level spell slots is tricky, and that is one of the reasons why it isn't ridiculously strong. You can still build characters around it (EK 7/Sorcerer 13 isn't bad; a 16d8+2*stat one-two combo, psychic and thunder or fire).
The real question you should be asking is:
1. Is the player in question likely to charop that character outside of the rest of the group's bounds?
2. Can the DM deal with any charop resulting?
3. Will it make the game more fun?
You aren't publishing a supplement, you are DMing a PC.