All the regular benefits of Free / Libre / Open Source software (No license fee, no fee per seat, no lock-in, access to source code, community, etc.). Ideally everything under LGPL / MIT / BSD style licenses.
Every one says they are interoperable. But sometimes, even with two pieces of software from the same publisher, there are issues. It's just a difficult problem. By reducing the number of possible interactions between the various components, we'll keep the complexity as low as possible.
Even if each piece of software works well, you very well could still have issues because "they don't talk to each other". A community which shares the same software interactions/combinations is in a better position to add missing features and make them stable and efficient. We'll have more eyeballs
Integrating all these components in one will increase usefulness (tight integration). Reuse of same components, in the same way will reduce support requests and headaches. By using less different software packages, this reduces training needs and increases efficiency.
Tiki Suite covers the most common needs. If you have specific software (commercial or otherwise) you need, nothing prevents you from using it.
You are starting a project and you need similar tools for your team. By going for the Tiki Suite, you can reduce the time you'll invest on making these decisions, knowing that other people with these use cases and criteria Came to a well thought out decision. Please see The Paradox of Choice: Why More Is Less and the Ted talk: Barry Schwartz on the paradox of choice