In short, there's no good reason to do them. Except of course when Ratchet is forced several times to participate in these little detours, either to eventually rescue a friend or collect enough of a certain item to earn a necessary upgrade. Clank has his own annoying mini-games, including one where a laser fixes fissures in time on various planets.
The other problem with gameplay comes from combat, which is generally a madhouse of fun with huge explosions and creative enemies. The problem is that the gun selection, while varied, is mostly made up of secondary weapons. That is, Ratchet only has a handful of weapons with which to fire directly at his enemies. Most of the other weapons such as mines, bombs and "Tesla" traps are proximity devices.
It may seem like a small complaint, but in the midst of a wild battle, running out of bullets for the pistol and having to try and hit a giant alien with a large electric bowling ball or bombs with extremely limited range is a study in frustration, no matter how colorful.
The game can be best summed up in a single character -- Mr. Zurkon -- who is actually a robotic weapon Ratchet can call upon assist him in battle.
While hovering over Ratchet's head, Mr. Zurkon fires off insults at his opponents, or Ratchet if there are no opponents around, as rapidly as he fires his laser blaster.
Completely action-packed, hilarious, quirky and clever, Mr. Zurkon is the perfect slice of "Ratchet & Clank Future: A Crack in Time" -- not without its problems, but far too fun and funny to ignore.
Like the little robot says, "Mr. Zurkon does NOT come in peace."