Sadly, I'm just not educated enough on subjects like this, but from what I understand from those that are, the Enlightenment Foundation Libraries are pretty impressive, combining low resource usage with advanced functionality and great performance. With the support from companies like Samsung, development has been given an impulse, leading to this beta release.
"These libraries were created after E16 to help with E17, the goal was to build infrastructure to aid development of rich graphical user interfaces that would be both fast and easy to use," the team writes, "They went through major rewrites over these past 10 years, but they have settled nicely over the past and now feel ready to be released."
The libraries were already pretty stable (my usage of DR17 a few years ago was already quite pleasant), so the act of stamping them with beta is done to indicate the team has hit API and ABI stability. This could lead to more widespread usage of the EFL.
All this also means that the final release of Enlightenment DR17 will arrive before the end of this year. "After 1.0 final the work force will be put behind Enlightenment DR17 and Elementary, the widget set, with the goal to have everything 1.0 final before the end of the year," the team adds.