it would be really bad if modem users were completely excluded from swarming, for several good reasons:

- first, modem users build a great part, i would say even the majority, of the peers in the gnutella network. by excluding them you offer this very great feature to a minority of your users only.

- second, modem users are the ones who suffer the most of too slow connections. they have so little bandwidth and they shall not even be allowed to use what they have? the average download rate on limewire for a modem user is about 2.0 kb/s. if you ever tried to download a 100mb file using a modem you know while it is absolutely necessary to support this group as well.

- third, if only highspeed connections support swarming, than highspeed and lowspeed connections are no longer peers. if a cable user wants to download a 700 mb divx movie, he might swarm from 6 connections, let's say 3 of them are modems. thus, he decreases the bandwidth of 3 modem users (who just have ~7 kbps). as these modem users are not necessarily running limewire you cannot guarantee that they are not used for swarm sources. however, they are not allowed to swarm from anybody themselves. therefore, swarming only helps highspeed users, while at the same time it could even harm lowspeed users.

- fourth, there is a social matter you have to regard: not everybody can afford highspeed internet, in some regions there is no way to get highspeed internet. therefore, it is support for better suited or better located people only.

- fifth, i hardly believe that there would be so much a loss of upload slots, because every slot would be used for a correspondingly shorter period of time. you just guarantee that the given connections are used up to their very limit. if this makes people to download more and therefore shortens the number of uplaod slots, well - i don't think one should decrease performance in order to make users download less. especially, you only make modem users download less - highspeed users can and *will* download more stuff, and they usually are the ones who download most. how many modem users are actually downloading 700mb divx movies ? i hardly know any, and if í do, they did it once and never again.

- sixth, if you are really concerned on not having enough upload slots on the network, than the first thing you should do is to remove the option to throttle upload bandwidth. many users just put it near or to zero in order to maximize download speed. eDonkey, for example, doesn't allow you to have more than thrice the speed for download than you have for upload.

well, these are my reasons. i strongly hope that you will enable swarming for modem users in the final release. other file sharing applications do it as well. you could make it optional and have this option disabled as a default for modem users, if you really think that it would help the network if modem users didn't swarm.
however i can't believe that you are giving such a great feature to the "upper class" only.