Some advantages I can see being based on ruby 1.8.7 is that now people will have a specific language to develop libraries for, also new people to ruby can have something specific to start with (rather than having to chose between 1.9/1.8).
Those that still want to move so quickly can still choose to do so and continue to use and support 1.9 they just might not have the same following as before.
Plus, it's important to note that the current draft spec actually supposedly covers the existing 1.9 implementation and all other existing implementations by excluding features that differ between them. From the Drafting Guidelines (http://ruby-std.netlab.jp/):
Second, we intend that existing implementations such as Ruby 1.8.7, Ruby 1.8.6, Ruby 1.9, JRuby, Rubinius, and IronRuby can conform to the specification without modifying them. There are some features which are not implemented in some of the implementations or are different among the implementations. Those features are excluded from the specification, or described as either "implementation defined" or "implementation dependent."