Tiki Suite should cover what 80% of the people use 80% of the time for a typical small or medium organization. The idea is to offer a well integrated set of tools for what most people need most of the time.

Open Source

All the regular benefits of Open Source (No license fee, no fee per seat, etc.)

Community

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.

Efficiency

By using less different software packages, this reduces training needs and increases efficiency.

Flexibility

Tiki Suite covers the most common needs. If you have specific software (open source or otherwise) you need, nothing prevents you from using.