I believe BI is connected to 1.4. IIRC, 1.2 came out before BI was released, and 1.3 was released at the same time as BI (1.4) came out. I know that many mods were made for vanilla 1.2.

However, it's been so long since I upgraded to 1.5 that I can't remember if it's possible to skip the 1.3 update.