Minecraft – Delete default profiles from new minecraft launcher (2017-)

You can make separate work directories for the launcher and start it like this:

java -jar ~/minecraft/Minecraft.jar --workDir ~/minecraft/user1/
java -jar ~/minecraft/Minecraft.jar --workDir ~/minecraft/user2/
#etc

(For Windows users, just specify the folders as you normally would: e.g. java -jar "D:\Games\Minecraft\Minecraft.jar" --workDir "D:\Games\Minecraft\User1\")

By doing this, you will get completely separate environments for the game, so each one of them will keep its own login data (you still need to press the "Play" button), but will also have separate worlds and resource packs, and the launcher will download/update the game separately.
However, that can behavior can be changed, especially easily on Linux. You can share the singleplayer worlds, for example, if you make a symlink to one environment's saves folder in other folder(s).

There seems to be no way to connect to a server with a command line parameter, because the only command line parameters the launcher accepts right now are: work directory, proxy server, force update. (use --help to look at that)

Yes

My earlier post was wrong, sorry, it was launching an old version and I didn't realise.

What you need to do for 1.6.1 is a little more complex.

First, you'll need to launch from the launcher. Assuming Minecraft launches ok, you'll get a load of output in the Development Console. What you need to look for is a line that looks a bit like this (yes, it's very long!):

java -Xmx1G -Djava.library.path=%APPDATA%\.minecraft\versions\1.6.1\1.6.1-natives-10107774015424 -cp %APPDATA%\.minecraft\libraries\net\sf\jopt-simple\jopt-simple\4.5\jopt-simple-4.5.jar;%APPDATA%\.minecraft\libraries\com\paulscode\codecjorbis\20101023\codecjorbis-20101023.jar;%APPDATA%\.minecraft\libraries\com\paulscode\codecwav\20101023\codecwav-20101023.jar;%APPDATA%\.minecraft\libraries\com\paulscode\libraryjavasound\20101123\libraryjavasound-20101123.jar;%APPDATA%\.minecraft\libraries\com\paulscode\librarylwjglopenal\20100824\librarylwjglopenal-20100824.jar;%APPDATA%\.minecraft\libraries\com\paulscode\soundsystem\20120107\soundsystem-20120107.jar;%APPDATA%\.minecraft\libraries\org\lwjgl\lwjgl\lwjgl\2.9.0\lwjgl-2.9.0.jar;%APPDATA%\.minecraft\libraries\org\lwjgl\lwjgl\lwjgl_util\2.9.0\lwjgl_util-2.9.0.jar;%APPDATA%\.minecraft\libraries\argo\argo\2.25_fixed\argo-2.25_fixed.jar;%APPDATA%\.minecraft\libraries\org\bouncycastle\bcprov-jdk15on\1.47\bcprov-jdk15on-1.47.jar;%APPDATA%\.minecraft\libraries\com\google\guava\guava\14.0\guava-14.0.jar;%APPDATA%\.minecraft\libraries\org\apache\commons\commons-lang3\3.1\commons-lang3-3.1.jar;%APPDATA%\.minecraft\libraries\commons-io\commons-io\2.4\commons-io-2.4.jar;%APPDATA%\.minecraft\libraries\net\java\jinput\jinput\2.0.5\jinput-2.0.5.jar;%APPDATA%\.minecraft\libraries\net\java\jutils\jutils\1.0.0\jutils-1.0.0.jar;%APPDATA%\.minecraft\libraries\com\google\code\gson\gson\2.2.2\gson-2.2.2.jar;%APPDATA%\.minecraft\versions\1.6.1\1.6.1.jar net.minecraft.client.main.Main --username %YOUR_USERNAME% --session token:%SOME_TOKEN% --version 1.6.1 --gameDir %APPDATA%\.minecraft --assetsDir %APPDATA%\.minecraft\assets

I've edited mine a bit to remove system-specific data, it'll probably start with something like
Running C:\Program Files\Java\jre7\bin\javaw.exe

Close Minecraft, but not the launcher. You will see some lines at the end of the Dev Console that read something like

Game ended with no troubles detected (exit code 0)
Deleting C:\Users\fredley\AppData\Roaming\.minecraft\versions\1.6.1\1.6.1-natives-11569750504590
Couldn't delete C:\Users\fredley\AppData\Roaming\.minecraft\versions\1.6.1\1.6.1-natives-11569750504590 - scheduling for deletion upon exit

This is obviously a problem! Make a copy of the 1.6.1-natives-11569750504590 folder (just make a copy in the same folder called 1.6.1-natives-backup in the same folder). Edit your command so that it uses your backup copies (just paste it in a text editor and find-replace 11569750504590 with backup).

Don't use my command, as it will not work. Account/session data is in the command so you must use your own copy as generated by the launcher.

Copy your entire, edited command (excluding Running at the start), and save it somewhere. You can then launch from cmd using this command at any time.

If you want to have an icon to launch Minecraft it's as simple as right-clicking your desktop, selecting New > Shortcut and entering this command as the 'file'.