That is a function that's been built in for several years. The reason the ultrapeer is dropped is because LW does not need it if there are no searches happening. Thus that ultrapeer can then be used to help someone else connect to the network. The dropping of the ultrapeer should not affect download speeds.

Edit: Check post #7 here http://www.gnutellaforums.com/downlo...leep-mode.html with sberlin's response of the reason.