You're right! They all do have a use. However, keep in mind that tons and tons of people have played competitively before you and more or less they've narrowed down some of the best ways to use each Pokemon. You can try to be innovative and come up with new ways, but to be completely fair a lot of the 'standard sets', the ones that lots of people recognize to be good and are used a lot, are really good. Movesets like Defense Curl + Rollout... well, while that may seem cool, there are a number of flaws! Firstly, what happens if they switch to a Pokemon that has a 4x resist to Rock when you start using Rollout? You're trapped, and then they can set up Swords Dance with Lucario while you're stuck doing nothing or very little to them. Secondly, Emboar is better off doing other things than boosting its defense with Defense Curl or similar; its stat build strongly leans towards offense, meaning you should try to smash things off the bat with heavy-hitting moves instead of mess around with things like Defense Curl + Rollout.
Everything does have a use, but keeping in mind what I said up there, there are standard sets for everything too. The biggest problem between some Pokemon being called 'useless' is not that they're useless but rather 'outclassed'. There are other Pokemon that do what they do best, better. For example, a very notable example would be the Blaziken-Infernape situation of last gen. They're both very very similar, being built as mixed attackers with good dual-typing and similar movesets, but because Infernape was just fast enough to outpace a lot of things, it was used way more because its speed was just such a huge advantage over Blaziken. Does that make Blaziken bad? No, but it made Infernape better.
It's hard to just 'back up their weaknesses' with the rest of the team. Don't think type weaknesses - everything has those save Drapion with Air Balloon, Spiritomb, Eelektross and Sableye. You have to think in Pokemon vs Pokemon weaknesses. Consider this: Swords Dance Scizor usually uses Swords Dance/Bullet Punch/Bug Bite with either Superpower (or Brick Break) or Roost. Skarmory takes very little from any of those moves and blows away the boosts with Whirlwind. This means that Skarmory is a counter for Scizor; Scizor can't do anything to it! So if your goal is to have Scizor sweep, you'll need a way to kill Skarmory or else you'll risk losing every time you see one. Do you understand what I'm saying?
You want to use Pokemon to their full effectiveness! Remember, standard sets are standard because tons of people have used them and figured out what their best and most effective sets are. If you have any questions about what I said or anything beyond that, don't be afraid to ask; I'll definitely help you out. :3