There are a few tradeoffs other than just the price/convenience with buying pre-shredded as opposed to blocks of cheese:
- dryness: I have always found pre-shredded cheese to be drier than blocks, probably because of the high surface area to volume ratio that shredded cheese has, compared to blocks. More suface area means more places for water and oil to evaporate from. (If you let hand-grated cheese sit out in the air for a while, it gets dry, too.)
- flavor: pre-shredded tends to be less flavorful than blocks. Why? Probably for the same reason as why it's more dry; more of it is exposed to the air, all the time.
- coarseness: cheese graters usually have two sizes of holes, one for fine strands, and another for thicker and larger strands. Pre-shredded cheese is typically not actually shredded, but pressed (or extruded) - if you don't like the granularity, you just have to make do. Also, the shape of hand-grated cheese is wide, slightly curved, and thin, wheras pre-shredded cheese is rectangular; from my own experience, the thinner cheese melts more easily.
So is it worth buying cheese pre-shredded? Sure, if you're really in a hurry, but otherwise, just apply a little elbow grease and grate yourself some from a block. Your taste buds will thank you.