Need is relative. The console manufacturers are all publishers too, locking their ecosystems provides them the following benefits, all of which they want, but not all of which you "need" or want.
- DRM. This prevents piracy, as others here have pointed out. This aims to prevent unauthorized copies, but it also gives them control over primary markets, and over payment systems.
- Quality. Locking allows them to review and approve content before publication. It raises the overall quality of the ecosystem, and reduces shovel ware and bad actors. This is good for publishers, because it increases safety and consumer confidence. This happens to be good for consumers too, because you get less crappy, less crashy software. (This doesn't meant you don't get crap... It just means that overall there's less crap.)
- Control. A locked ecosystem controls many aspects of the second hand market, even more so as downloading takes over hard copy. It has in the past also allowed publishers to have proprietary advantages.
- Ratings. They review content for age appropriateness. It could be self reviewed like the Google play store, but they prefer to be certain. Especially important for Nintendo.
Anyway, they would not be able to reap all the benefits if it were open, and they're in charge, so I guess it won't happen soon. I slightly disagree with the premise that locked vs ecology is the dichotomy. The locked systems do have ecologies, they're just probably different than the one you want, and unfortunately they shut out us makers and experimenters, relative to the platform itself, so they are ecologies of pure consumers.