The following is based on a decompiled version of Minecraft 1.9 using MCP 9.24 beta.
This has very likely only an effect on the title screen.
When you switch to title screen the garbage collection cannot collect everything from the world you had open before because Minecraft.pointedEntity still references it.
To reproduce, simply quit to the title screen while looking at an entity. The field will not be set, and a WorldClient instance will be leaked.
This happens because the field is not set to null on the world changing, and EntityRenderer.getMouseOver only sets it to null when there is a render view entity (and a world to render).