This was to allow for additional control of mob spawns for spawn control mods such as [Mob Spawn Control]( http://www.minecraftforum.net/topic/1553186-15forgemodloader-mob-spawn-controls-control-spawning-increase-spawncaps-spawn-attempts-mocreaturesextrabiomes-support-updated/] . Unlike the spawn properties in SpawnListEntry the more commonly needed getMaxSpawnedInChunk is not exposed). 
The advantage/need of the non-creature mods having access is to allow a single UI to tweak and balance worlds utilizing several creature mods. The balancing with respect to several mod combinations cannot be balanced from the single mod authors perspective and the single UI is typically outside the scope of the individual mods.

It could obviously also be used to tweak vanilla spawn distribution.