When a Profile with the Participant Role field is included in online Event Registration - and a value is submitted - that value should over-ride the default role for that event. Currently we ignore the profile field value and use default role.
Either check out this user-configured event on the sandbox (if it's still there):
1. Create profile and add Participant Role field
2. Link that profile to an event's Registration form
3. Register for event and select a role other than the default role