You'll need those ultrapeers so that you can search! You'll find that connecting to more ultrapeers, as in LW Pro, can give you greater search results.
Can't even remember where I got this info from (ages ago) but it explains pretty well:

" Ultrapeers are the backbone of the Gnutella network. An Ultrapeer handles most Gnutella traffic, shielding its Leaves from this traffic. Ultrapeers accept connections from the Leaves, giving them access to the network and handling their requests. Ultrapeers also connect to each other, bringing the network together, Leaves do not connect to each other. Leaves commonly do not meet the requirements for being an Ultrapeer.
If Ultrapeers are the backbone then without them all peers would be the same and form a much larger skeleton. Messages such as queries would have to travel further to reach the same number of people and so achieve the same result, that would take up much more bandwidth. Ultrapeers are chosen from the more capable computers and are able to form more simultaneous connections than many computers can, by doing this it takes fewer 'hops' for a message to reach it's target."