What player, whose PC has access to fireball, DOESN'T try to maximize targets in the AoE? Minis or TotM, I'm going to try that. Whether the combat state is maintained on the table, or in the DM's head, I'm going to (try to) get that information. If it's on the table, I can do that when it's not my turn much easier. If it's in the DM's head, I have to ask him how many I can get without affecting my friends, how many I can get with affecting my friends. This is why I think TotM is inferior for all but the most trivial of combat encounters, regardless of the level of player/DM trust.
And for you, it may be inferior. But, not everyone is you.
For example: Not everyone wants to count squares as part of their roleplaying experience, even if that means a slightly more effective fireball. For another example: Not everyone even feels that such ability to find the optimum placement for a fireball is particularly plausible, as most people aren't really good at estimating distances and sizes by sight.
The trick is not to identify one or the other as overall objectively better. The trick is to find what purposes and players each tool serves best, and to adjust to your own group's needs.