"fast" . . . is relative. If UP's are needed, then maybe a 50 KB/s connection would be useful as an ultrapeer. But if there are more and faster connections available, then the 50 KB/s would probably not be needed. Currently, even a 500 KB/s connection can be passed over because there are many more faster nodes around.

The advantage of running as an UP is that they handle much of the traffic and messaging necessary to keep the leafs connected efficiently. So, you can contribute bandwidth to the network if you don't like sharing popular files. There is not much personal benefit. Searches for rare content will theoretically reach more of the network, but even a Pro leaf will search further than a Basic Ultrapeer. Viewing incoming searches is much more interesting though, since you see all the the search terms passing through your connection the network. As a leaf, most of those are are not sent to your machine to reduce the unnecessary traffic and bandwidth.

So, bottom line: for most of us home users, just let the code decide and don't worry. For those who like to play and understand the software, running as an UP is interesting.