if you prefer the sound without a sub, then it is not worth buying. Remember, a sub plays only low bass whereas floorstanding speakers play the entire spectrum of sound. you are right when you say that a sub is more necessary with bookshelf speakers. With the M60's, it's more of a complement to the system, but is particularly usefull when watching 5.1 movies where the subwoofer has its own channel, and all sound below a certain point is sent to the sub.
but, it's really up to you if you want it, and if you can spend the money.