You act like it would take more than a minute or two to implement. There are real reasons to take into consideration.
All you do is set a timer that checks how much it has received since the last timer interval. If it's less than the set "download speed limit", it sends the dcc "ack" so the sender continues the xfer.
Mind you, this fix would only work with clients that follow the standards for the dcc protocol (choking the send until it receives an ack) and it would not be a 100% precise slowdown since some people set their DCC packetsize differently. Perhaps the confusion over why it may or may not be working at times is the reason for not adding it.
It may also be possible to do the checking on the data-receiving level, but I am not sure how safe that would be since there might be an excessive transfer overhead.
