It is not a duplicate since the arguments "-DsocksProxyHost=localhost -DsocksProxyPort=5678" work until 13w39b.
Moreover, it is not a launcher issue since minecraft (the game) is run with the correct arguments.
The only thing that change is the Minecraft version.
And, I can reproduce the issue without the launcher using either --proxyPort --proxyHost or -DsocksProxyPort/Host :
MC-38872
Low
It is not a duplicate since the arguments "-DsocksProxyHost=localhost -DsocksProxyPort=5678" work until 13w39b.
Moreover, it is not a launcher issue since minecraft (the game) is run with the correct arguments.
The only thing that change is the Minecraft version.
And, I can reproduce the issue without the launcher using either --proxyPort --proxyHost or -DsocksProxyPort/Host :
Minecraft 1.7.2 does not take into account the proxy settings.
Plus 13w39b (and previous) uses -DsocksProxyHost/Port but ignores --proxyHost/Port.