I believe the AI does take pledges to protect into account when deciding whether to demand tribute or declare war on a city-state.
I don't really think so. What pledge does for tribute is increase the CS "resilience", so yes, in a sense, it will make AIs less likely to demend successfuly, however, on higher difficulties (especially immortal / deity), they will all ask for tribute at some point, even the most peaceful civs will
It's true the 5 additional turns are not wortheless, but again, on higher difficulties, doing CS quest is not easy. Killing a barb camp might be possible if it's very close to you (the main reason why i rarely raze nearby camps), but then, even camps 4 tiles away from your cap can be razed by AIs even before a CS asks for it, all the tech/faith/culture quests are pointless early with AIs big boosts. This leaves NW quests (rather unpredictable) and finding other civs quests.
Anyway, as i see it now, it might be worth pledging in the early game, but it will become pointless pretty soon as you advance to later eras. Well, it's better than the free friendship that made consulates a no-brainer for every game. At least now, opening commerce or aestethics (or even honor/piety as a second tree) will be a valid option. Patronage will still probably be valid, but going further than just consulates.
I just hope it's only a quick fix, and they will work on a better pledge system, or maybe even re-think the whole CS relations system, for a future big patch.