Leaf blocks placed by the RFTools builder will decay when one is updated and there is no wood nearby. If the decayable bit is set to false, like when a player places a leaf block, structures created with the RFTools builder will be more permanent.

While building a 181x73x181 ellipsoid, I had the whole structure decay away after breaking one block. The decaying leaves dropped 4000 entities (saplings and apples) that dropped the server to 2-3 TPS until the items decayed.

You can see the decay in actions here: https://youtu.be/5SC2H2Pu7ko?t=1m56s