I would say "Hell no!" but I run systemless games.
On a more helpful note, if you know your system well, you should be able to roughly guess what a monster's states should be given a general description of its powers/hardness. Levels were originally designed as a short cut to describe just how hard something was: At Weathertop, the Witch King would be level 30, Aragorn would be level 15, and Frodo level 2. Sure, you could argue that Frodo was level 1 or 3 or 4 but overall his low level gives you a fair idea of what he is capable of. The rest can be filled in so that the characters are having a harder/easier time as dictated by the story.
As long as your players know that you are fair and are telling an interesting story they can influence and take into uncharted territory, then you are should be fine. As for special abilities and so on, feel free to use some that make it a more interesting combat. A character just set the orcs on fire, well, some of them had dynamite. BOOM. A character has a nice toy he can use, make sure he uses it on something big and important. Referees are here to tell a cool story featuring the characters. The players are there to shape and mould that story
For example, 3 orcs attack Aragorn. Let's assume that the referee wants Aragorn to shine in that combat. It's just to show how hard Aragorn is to all the other players and a few NPCs. Aragorn's players does not know this. So, he rolls his attack as usual: superb result. Fine, the first orc is cut in half at the waist. Gore everywhere. Awesome thinks Aragorn's player. Second attack is in the low range for Aragorn's level. Fine, the orc gets a slash across its face and is now blind in one eye. Etc...
For example, now Aragorn is fighting the Witch King. Aragorn rolls well but not brillantly. The result is now that the Witch King parries the blow with apparent ease. Aragorn is in trouble. It's going to be a hard fight... And so it should be!