Ingame wise, the starters is an indifferent bunch. You survive without them but they can be a great help. Overall you can clear the games with anything by over levelling, but the starters can be nice to have because they really aren't useless.
Competitively, the starters stand on very different ends of the spectrum. We have Blaziken and Greninja who dominate normal play, but also Meganium and Samurott who are worse than a tomato for fending of hungry bears. However, no starters should be used in competitive battles.
"But Uncle Flafpert, you just said Blaiken and Greninja were really good you
dingus."
Yes, they are good, under perfect conditions. Both Blaziken and Gerninja need their Hidden Abilities to function, and they all need IVs, Natures and Egg moves to properly function, even Samurott and Meganium. And the starters you receive ingame lack all of these. They are random blokes, imperfect specimens, bad Pokemon, the whole bunch. Sure you might get really lucky and get a Charizard with 6 perfect IVs and golden talons and its own luxury yacht, but this is insanely rare. In all cases it will be easier and better to breed your starters to get Pokemon that can be used in competitive play, where you can plan ahead and make them the way you want (you are the Arceus now).
So in short, stick with them for breeding purposes but don't use your starters in competitive battles (unless you get really lucky and strike a good one).
