The storyline grinds to a shuddering halt as soon as the credits roll. Here's Looker to round up the Seven Sages in a game of hide-and-seek, and here's to Ghetsis escaping off-screen and the Shadow Triad ****ing off with no resolution to your conflict!
Keledo, Meloetta and Genesect have really short bonus events that happen if they're already in your party - there's no events that actually involve encountering them or anything they do (it's impossible to encounter them in any way other than a give-away, they don't have event items or event locations). Kyurem is sitting around chilling and maybe eating somebody at night but you never actually see him do anything.
The Pokémon Musical is very, very bare-bones, the Battle Subway is missing most of the Frontier's features, they brought back the Diving mechanic for a single small area that just holds hidden items, and generally everything that you encounter post-Elite 4 is lacking. It's even indicative of re-used content, in a way - Looker appears from nowhere, you get a post-game boss in Cynthia (her sprite is an edited Platinum sprite, while her theme music was barely changed at all) and the Diving function was recycled out of nowhere.
Of course they would release an unfinished game - if they reach the project's proposed deadline (September, the same date that every Pokémon game since Emerald came out) and the majority of the game is in a playable state, and the storyline makes most sense, they are not going to keep working on it to polish off the aforementioned rough bits when they have a publisher waiting to send games to store shelves. Unfortunately, it's an all too common story of managing projects in the games industry, and essentially it's what all software development boils down to - do as much as you can with the time and resources you were given to do it, and don't expect to get extensions unless you've messed something up really badly.
They simply shelve the more complicated things they didn't have time to implement into the game (Something like the Following Pokémon from HG/SS that didn't make the cut, for example), tie up the loose ends in things they originally envisioned to be grander, and leave stuff lying around that's indicative of later plans (Like unused legendary Pokémon)
There's evidence of unfinished content in almost every video game that's ever been made. Pokémon is one of those in a unique position that they can go back and finish that content before selling it off two years later. There's also been a lot of discussion about Ibe's comment when it was said. The most important thing to realise is that these guys have no qualms about lying to us - what idiot would tell people that they're going to make another, better game when they haven't even finished selling the first games yet? (In defence of a sound business decision that's basically the least damaging way to respond to that question)