One reason why ForgeGradle has been added is because it is simpler and easier to use!
another is for people with OCD like me with ForgeGradle you don't have loads of .bat, .sh or .py files you just have two files!
I'm sure there are a lot more reasons as to why it should be used but to be honest the old way to build and decompile was better because it never used to take up 1GB+ of memory for all of the mods I have!