Not sure if this is done on purpose, or is an issue.
I've tried both the switchyard-installer-0.3.Final and 0.4-SNAPSHOT, it produced the same result.
When I ran the ant against the installer folder, and choose the 'n' for the forge installation, it still asked for the forge home.
[input] Would you like to install the SwitchYard Forge Module (y/n)? ([y], n)
[input] Please enter the path to the root folder of your Forge installation:
Hmm, this seems to be a regression. Forge is not required and you shouldn't have to enter Forge home if you select no for forge plugin installation. Can you file a JIRA and mark the fix version as 0.4?