I feel like optional filler is essential to any game, as it increases the time the player can spend immersed in the world and marveling at its lore and environment. Sure, that is dev time that could be spent in the main story, but we're hackers, and we're not being rushed by external deadlines and such. Sidequests, revisiting towns to find hidden corners with HMs, etc. Even putting some continuity in by having certain NPCs in different towns know each other can help flesh everything out, even though many would consider it filler.
You're not just telling a story and handing the player powerful Pokemon to beat it up with, you're crafting a world that people live in. Filler is an essential part of that.
Now, I'm not saying that there should be mandatory filler everywhere, because that's awful, but (optional) filler should definitely be there for the player if they want to explore it. If the player is the type to check every nook and cranny? Boom. You've just lengthened their playing time and upped their enjoyment. If the player is the type to blaze through the story like a speedrunner? Boom. You've just given them postgame to do that's separate from any actual postgame you have planned.
Seriously, filler is awesome, and it makes me sad when I play through a hack and literally the ONLY thing you can do is follow the story and nothing else.
By all means hackers, when doing your hacks, focus on the story first. But don't forget to go back and flesh out some areas so players have a reason to go back to the towns and routes you no doubt spent hours creating. You owe your work, and your time, at least that. It's obviously difficult to plan ahead when creating areas and crafting your story. But it's not difficult to set a goalpost for yourself (like when releasing betas) for the main story, and then taking a day or two after that's done to take a step back and flesh out earlier areas now that the story and region is more solid.
