Engineers hate it for an obvious reason: "what if that feature doesn't work or live up to the hype?" Fear of under-delivering is a healthy fear for your engineers, but it is also irrational. If the product under-delivers, you've got bigger problems, usually, than the fact that you made bigger promises early. My advice: deliver! Also commonly, engineers can't understand the need for pre-launch buzz and hype.
Marketers want pre-launch buzz and hype. Why? Three simple (and one sad) reason.
1. Sad reason: because many don't know how to add much value in "pre-launch" phase. It's sad because there is much that can be done: competitive analysis, pricing studies, etc. Those are not the sexy parts of marketing however, so they get "not done", and pre-launch becomes the exciting focus.
2. Simple Reason: Economics. Increase demand! If you can get an early start on "creating unfulfilled demand" than when the product does launch, there will be a rush to buy and be proud you got one!
3. Simple Reason: Measure Demand. If you do the buzz marketing, and get no buzz... you may have an issue (or need to tweak your marketing/positioning).
4. Simple Reason: Press. Getting press involved early means getting 2 stories instead of just 1. Without a pre-launch story, all you get (often) is the "launch" story.
Now get out there and "pre-launch" something!