So it seems there actually may be real benefits of using FPC 3.0 and Lazarus 1.2.x together, if I got that correctly...
You mean Lazarus 1.4.x, not Lazarus 1.2.x. The former is already forked and RC1 will come soon, 1.2.x will not be updated any more.
Otherwise you have a valid point. FPC 3.0 will have a huge number of improvements and new features. It would make sense to use it at some point for Lazarus 1.4 series. It has been done before, IIRC Lazarus 1.2.0 was released with FPC 2.6.2 but Lazarus 1.2.2 with FPC 2.6.4 (or something).
Lazarus 1.4.0 must use FPC 2.6.4 because 3.0 will not (likely) be released yet.
We try to get 1.4.0 out rather soon while FPC 3.0 testing and stabilization will take longer. When FPC 3.0 is out, Lazarus must be tested more with it.
After that maybe 1.4.2 could use the new FPC. Let's see. It has not been discussed anywhere.
Some visually apparent changes of Lazarus 1.4 are listed here :
http://wiki.freepascal.org/New_IDE_features_since#v1.4_.28to_be_announced.29It also has tons of smaller improvements and bug fixes.
People who use trunk knew this already, yes.