In the context of min-maxing the ability of your character to kill lots of monsters, it is indeed useless unless you are a caster. Although previous versions had various supplements that allowed you to use Int for combat, I have no doubt that similar things will soon materialize for 5e if the system proves popular and profitable.
However there is more to roleplaying games than just combat. Analysis from a char-op perspective glosses over these aspects because they are less straightforward to optimize, but in practice I've found them to be a very big part of the game. In fact, combat is cumbersome due to the many die rolls and complicated rules, and especially new players easily get confused - I've seen non-combat elements take a central role in DnD games as often as not.
The most obvious point is roleplaying. If you want to play a brilliant professor, knowledgeable sage, genius inventor or some other kind of witty know it all, it would be very awkward to do it with an Int score barely higher than the village idiot. If your goal is to kill lots of things good, the charop conclusion from this is that you should not play a "smart" character because it's suboptimal. But if your goal is to experience an interesting narrative, you might find that a high Int character makes sense despite the drawbacks.
The second use is non-combat challenges. Again, if your DM runs games that resemble multiplayer Nethack, with monster after monster thrown at you in never-ending dungeons, yes Int is not very useful. But if I am DM'ing a party that has high Int characters, I will try to provide puzzles and similar challenges (mechanical contraptions, deciphering ancient scripts, references to legendary or historical things that provide clues on how to navigate the dungeon or find important items) which basically require the high intelligence to overcome.
Also, in almost all games I've played, during the "preparation" or "intermission" phases (when you're in town preparing for the next adventure) high-Int characters have always had a chance to shine. For instance examining official records to find corruption or locate criminals (especially when that is the stated goal of a quest), researching through libraries the weak spot of a boss monster, finding shortcuts to a quest goal, all become much easier if you can pass a few Int checks reliably.
In parties with less than perfect mutual trust, the high Int character is able to perceive critical quest-related information, and then use this as a bargaining chip against other characters. As you say, many uses of Int can be dealt with by just having a single high-Int character do everything for the benefit of others... But think realistically: If I happen to be one of the handful people alive who know how to activate the powerful artifact we are looking for, would I tell you? Maybe I don't like you too much, and I'll refuse to share my knowledge unless you treat me in a way that appeals to my ego. Or maybe I'm scared that once you know what it looks like, you won't need me anymore, so you'll abandon me and split my share of the proceeds, so I'll refuse to tell you as insurance against your disloyalty.
In reality, intelligent people are respected for various reasons. Many DnD players will tend to naturally transfer this respect into the game. Even if their character does not personally respect an intelligent character, they will at least be aware that they should. This gives a lot of leverage in social situations. Both due to pragmatic reasons, as the above paragraph, but also just the fact of being intelligent can be used to persuade other players (through persuading their characters) that you should be the leader of the group and decide important matters.
In fact, the advantage of high Int is amplified in the game vs. reality, because players are often very unfamiliar with the game world, much more so than even an ignorant and stupid inhabitant of the world. To be able to even function, they need exposition, which can happen through the DM just flat out giving the information out of character, or the high Int character happening to know this or that bit of insightful trivia that explains why things happen the way they do. The players then become dependent on this character to make sense of their unfamiliar (to players, though not to the characters) environment.
People don't seem to like optimizing non-combat abilities, presumably because they depend so much on what sort of campaign the DM decides to run. Granted, for combat it also matters a lot what sorts of enemies you will actually fight, so I think it's more that people can fool themselves into thinking they know what they'll encounter by pretending the monster manuals are representative, rather than a genuine predictability of combat. In any case, if you did try to do it, the result would probably be a character who is intelligent, wise, charismatic, educated, upper class (ideally aristocrat or royalty), famous and cherished throughout the game world. The difficulty of combat is moot to such a character, he can have hordes of his fans protect him, he can talk kings into giving him elite regiments as escort, not only does his status afford tremendous power, but much actual adventuring can be delegated to various lackeys and sycophants. In such a game, combat optimized characters will be the ones scrambling to maintain their relevance. For such "non-combat" optimization, intelligence is a vital attribute, both because it can make you pass many useful checks, but also because it is a pretty good way of justifying your high social status (two obvious examples are the respected scholar and talented merchant).