Sentinel was definitely a very situational job. It was useful to have a paradigm that included sentinel (usually COM/MED/SEN) when you needed it.

I was a bit more annoyed though at Sazh. I can tolerate a role being situational, especially given both its users are both excellent characters battle-wise. But Sazh felt like a very situational character: His only good role was synergy. His strength and magic, well, when your strength is below that of the game's main healer (Van'ille), something is wrong. Hope eventually got all of Sazh's synergy, just Sazh gets certain Synergy (such as the really useful haste) a lot sooner. But, even so, the only time I used Sazh was when I couldn't have healing and synergy compounded on the same person (i.e. Hope), which only happened on a few occasions. Otherwise, I just used Hope and survived on Lightning's healing.