Subabul is a leguminous tree but an invasive one in Palakkad,Kerala climate. You can see 100s of small Subabul trees under a grown up tree, that is why it is called an invasive species. Also after growing cutting and removing them is practically impossible, if you want to plant some thing in that area. Also the biomass generation by this tree is not much, since the leaves are very small. Goats and Cows like the leaves, so can be fed to them. So in my opinion, if you plant them and leave it unattended it can cause lots of trouble, if you periodically cut them before seeding, then it is fine. But leguminous trees like Glyrecedia, Pongamia etc are OK, even if you leave them for an year.