The main answer to your question is "when you want to play a character who is more naturally strong than naturally dexterous." But you're looking at an optimization strategy, so that's not a complete answer.

From a purely optimization standpoint, I'd say:

1. You don't explicitly call out in your math favoring an extra Dexterity level that the extra die is worth 0.4^2 = 0.16 damage. It reads like you might be operating from the assumption that it's the same 0.4 you get from a Strength die.
2. Potence and Celerity aren't available to all characters; if your question is specifically about Vampire and its variants, I'd suggest changing the title of your post.
3. Strength is used for things besides damage. It's hard to work that into an optimization algorithm, but you may need to kick down doors, lift a car off someone or whatever else, which Dexterity won't get you very far at.

Really, though, all of these are mitigating factors toward the basic fact your question is pointing at: in the classic World of Darkness rules, combat is far more ruled by having a high Dexterity than anything else. This is a pretty widely-accepted interpretation of the system, that nearly any time the system has an opportunity to make Strength or Stamina relevant, there's some shortcut that makes Dexterity even better at it. You probably won't find many people arguing against that conclusion; whether that's a feature or a bug, however, is pretty hotly contested.

The main answer to your question is "when you want to play a character who is more naturally strong than naturally dexterous." But you're looking for an optimization strategy, so that's not a complete answer.

From a purely optimization standpoint, I'd say:

1. You don't explicitly call out in your math favoring an extra Dexterity level that the extra die is worth 0.4^2 = 0.16 damage. It reads like you might be operating from the assumption that it's the same 0.4 you get from a Strength die.
2. Potence and Celerity aren't available to all characters; if your question is specifically about Vampire and its variants, I'd suggest changing the title of your post.
3. Strength is used for many things besides damage. It's hard to work that into an optimization algorithm, but you may need to kick down doors, lift a car off someone or whatever else, which Dexterity won't get you very far at (but Potence will).

Really, though, all of these are mitigating factors toward the basic fact your question is pointing at: in the classic World of Darkness rules combat is far more ruled by having a high Dexterity than anything else. This is a pretty widely-accepted interpretation of the system, that nearly any time the system has an opportunity to make Strength or Stamina relevant, there's some shortcut that makes Dexterity even better at it. You probably won't find many people arguing against that conclusion; whether that's a feature or a bug, however, is pretty hotly contested.