LaunchAgents that are configured using SMJobBless are intended to be non-gui services. They run without UI in the background, and don't run as the logged in user.
If you want a gui, you're going to have to use SMLoginItemSetEnabled, or manually install a user LaunchAgent via a .plist with the "LimitLoadToSessionType" = "Aqua" property.
Helpers launched by SMLoginItemSetEnabled run as the current user, so they can be terminated by that user. The process will get relaunched if launchd detects that the agent went away because of a signal or due to an abnormal termination. If it terminates due to a clean termination it will not be restarted until the next login of the user (by clean termination we mean terminating with an exit code of 0, which will not happen if the program is killed e.g. via the activity monitor. If the helper has an option to quit, and that option causes it to exit with a 0 return code, then it will not be restarted, which would be considered good form by the application.
It's a stylistic thing - a user cannot disable the agent without interacting with the command line which would be considered poor standing. There is no issue displaying a UI from a LaunchAgent. As long as it's registered with the appropriate keys, it will be launched in the user's gui login context which means that it can be fully interactive. I have several applications which act like that - TunnelBlick, 1Password (this is via the LaunchAgent plist) and they only put up a menu bar item on startup.
If you're just shipping an application - e.g. via the app store, then there's no way to have anything automatically start without some interaction by the user - you have to launch the main app to get the helper to be enabled. The helper login item can be launched manually without ever launching the main application; but it won't have the 'launch at startup' magic enabled. If you can register a LaunchAgent plist, then you can get the launch at startup & starting it up immediately behaviour.
For uninstallation, however, LaunchAgents require the removal of the .plist, and interacting with launchd to disable it, items registered with the Service Mangement framework will naturally vanish once the app is dragged to the trash.
IMHO, (bit soapboxy, bit opinionated) If you're shipping a standard app with a helper, then rely on first launch before configuring such things as helper start on login; primarily because it keeps the user in control. Autoinstalling startup items is one of my pet peeves with windows applications (so many status bar items...).
