Your proposal would give the player a choice between two balls. It doesn't matter if they are both in play at the same time, they both have the potential to be the ball in play (you know the lie and situation for both, and that can influence your decision), and that is a scenario which the rules abhor. It allows you weigh your options based on things like current lie, potential drop areas vs. lying 3 in the fairway, etc. It diverges too far from the basic principle of playing one ball from tee to hole.
The current rules don't "abhor" this scenario. You have 3 relief options for an unplayable lie. Each option allows you to evaluate the best lie, distance, angle to the pin, number of strokes, etc etc.
This proposal does not change anything already available to a player.
Please provide a situation that would give a player an advantage over the current rules?
You aren't reading what's being written. You have 3 options for the same ball under rule 28. With your proposal, you have 2 of those options for one ball, but you also have to option of choosing your provisional ball for which you already know the situation. This is what the rules can't tolerate.