Grease has no current use in the game, except as a joke spell. In fact, it seems a little odd for turbine to put the work into creating a spell that has no use in the game, but I guess thats the case with a lot of DnD spells e.g. Death to Undeath, Symbol of Stunning etc etc.
If they were to change it to become enemy-only and heightenable, it would probably become more useful than web due to the way grease works. Both are reflex saves but you make a save for any movement in a grease, whereas web only has a save when you enter it, or re-enter it. Grease is also a larger AoE.
I disagree on the humour aspect though. An extended grease on the spike trap just above the ladder in von5, along with selectively dancing peeps as they cross the spikes can be hugely entertaining.