While the limit is a bit artificial and not in the human players favor, unless you are already conquering the world in which case you probably don't care about the city-states anyway. There is a difference probably tho as far as the city-states would be concerned in regards to being offered protection from a super power or to be offered protection from the civ with the weakest/lowest military_strength_score. Still it's nice to have friends, just that the reward might somehow differentiate depending on who you are, also then perhaps the blame if something happens to it while under your "protection".
I'm not really sure what it would infer if the limit was dropped to say 40%, as suggested. I imagine that more or less as many civs as possible or could pledge would do so and there would either be an increase in war or world pacification cause nobody wants to piss the others off by attacking a city-state (or trying to bully them etc).
I might not remember this correctly but isn't really all that the pledge does is lower the drop rate (or decay) of your influence with them? Possibly also triggering events when others interact with them? There is no actual real reward, they are not paying out extra yields or some kind of protection fee? It might be more worth it if say you actually got some kind of reward from it while it is maintained beyond the feelz of goodness. So they like you a little more, or hate you less or whatever. I can't really recall a game where this was a relevant factor for the game. This sort of almost links to that previous topic on how do you make taking/liberating/whatever city-states be more significant. Lowering the pledge limit doesn't really do much then. So more people will do it.