The issue is copyright - although we know that things like PDANet need to be sideloaded for use on some phones, for example, the intellectual property in that app means that we can't just go posting it anywhere we please.
We can allow posting of apks that are either open source (such as standard Android components) or developer efforts (with links to XDA or as found in our own developers' forums here at AF). Beyond that, links to the Market or trusted alternatives like AppBrain.com or developer websites are really all that we can allow and stay within the law.
Anything else is to condone the opportunistically self-serving, only seeking to profit by the theft of others' work with nothing more than the flimsy excuses of entitlement.