rmontiago: Up until wizardry 8, Fighters are a kind of very basic class with very little incentive to stay tied to it. Thieves are like that as well. Same for straight Mage or even Priest. There are few reasons to remain in the "lesser" classes.
There is the fact that, in earlier games in the series, these basic classes level up faster.
In Wizardry 1-3 and 5, if you want to max out a character's number of attacks, it will take less XP to raise a Fighter to level 45 than to raise a Samurai or Lord to that level, or a Ninja to level 40.
Also, if you want 9's of all spell levels, raising a Magie or Cleric to level 21 is more feasible than raising a Lord to 24, or a Samurai to 30, or a Bishop to 33/36 (yes, that's how high you need to level up a Bishop to reach all 9's). Even if you don't level up that high, level 13 is much easier to reach, and only Mage and Cleric get 7th level spells at that point.
As for thieves, their advantages (as a final class) are:
* Faster leveling (level affects disarm chance, saving throws, the chance of certain spells (status spells mostly, but also CORTU/BACORTU in 5) working, as well has now much you can cast HAMAN/MAHAMAN (costs a level to cast)
* Thieves are better at inspecting traps than Ninjas (for Thief, chance is AGI * 6; for Ninja, only AGI * 4)
(Worth noting that classic Sir-Tech Wizardry games are often designed to be beaten around level 13. (Contrast this to the likes of Wizardry Gaiden 3, where your endgame level might be in the 20s, and the post game expects you to reach much higher levels.))
As for Wizardry 6:
* Fast leveling still does matter in the long run, as XP required per level does still depend solely on class after a certain point. (Note that this changes in Wizardry 7, where XP per level is the same regardless of class or level past a certain point. Then Wizardry 8 changed it again and made XP requirements never stop growing exponentially.)
* With that said, note that base miss chance stops improving after level 20; once you reach level 21 it can never improve again for that character
* Valkyries level up faster than Priests, for some reason. I believe Bards level up at least as fast as Mages. (Strangely, I note that Wizardry Gaiden 3 copied this, even though those do appear to be balanced issues, along with Rangers being not that good.)
(Wizardry 4 is so unlike the rest of the series that this discussion doesn't make sense there.)