The limit is there to avoid causing overhead for transfers (ie lost of upload speed for communicating to many hosts)
Take it like that 100 hosts swarms between themselves. They get 8 dl sources and 8 uploads slots taken, they share as much as they give. Now, everybody gets the same speed, ie 80KB/s.
Now take case 1: everybody downloads from everybody, 100 sources, 100 uploads slots, here just to maintain active connexions for transfers up and down transfers (http consume badnwidth to maintain transfers), you would loose like 5-10KB/s of real transfer speed both in DL and upload, making transfers SLOWER than my first exemple. Not to mention that without caching of transfers in RAM (higher ressources consummed both CPU and RAM), you hard disk you scream like crazy and lower its life.
Anyway, its a multifactorial answer and there are many other factors, but there is an efficiency limit to increase # of hosts you dl from. This limit is higher as pipes and computer get better (previously FW was limiting at 6 sources), but this will increase as technology improves.
Hope you understood
Ciao