I honestly don't believe in gritty reboots, or reboots in general for that matter. Now if it's something like the aforementioned Teenage Mutant Ninja Turtles and Grimm's Fairy Tales, where the original work was dark and gritty, but subsequent adaptations have watered it down, then yeah, a reboot with the express intent of bringing the adaptations in line with the source material is entirely justified; just keep your dark, gritty reboots out of my light, hopeful series, and we'll get along just fine.
[sub]For the record, the original TMNT comics rule, as do the uncensored fairy tales; I don't like it when a series is so dark that it's depressing, but I quite enjoy an occasional splash of darkness, as long as it isn't the only flavor of fiction available.[/sub]