The actual "core-default" Elgg friendship uses the same model as Twitter: the relation is not reciprocal and it's not required to confirm a friend request. It is really "following" rather than "friendship". The interesting thing is that there are plugins to change it to the reciprocal/confirmation model of Facebook/LinkedIn.
The following relationship is more appropriate for certain contexts - eg. a corporate network, educational institutions, etc, and the friendship relationship for others, but basically ther's use cases for both. There were three options: use reciprocal friendship in core, and try to remove it via plugins when it's not needed, use the following model in core, and add recipocity via a plugin (current method), or support both in core with some option to enable/disable each. The second one was chosen because the more features that are pushed from external plugins to core increases code overhead and maintenance though - which slows further development.
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