I understand that people want their favourite plugins as core features, however it's not always in MyBB's best interest to do so for everyone. Adding features that the vast majority may not use means that we've invested time in coding, testing and tweaking a feature that is going to get dusty on most boards.
It may also mean a larger file size and slightly lower performance (more fields in the database require slightly more memory, storage space, etc.) as well as extra code to be executed.
We feel that leaving some things as plugins is easier for everyone because of this.
Thinking about it now though, a rework of it may be more useful, so it can be adapted to be used as a like/thank you/reputation at the discretion of the admin. Not exactly sure how, but if we wanted to incorporate the 'like' idea, that's probably the only way I'd see it done.
That's just me personally though.
Edit: Also, I see you've seen this thread:
http://community.mybb.com/thread-99746.html
Where it states we will have more SEO.