I came across SOAP method of authentication described in the below link: https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_calls_login.htm?search_text=login
Using the sessionId obtained from above call, can be used as an "access token", to potentially access all the secured resources. For now I have just tried to get user profile using it, and was successful. I will further try to use Bulk API.
So wanted to understand if it is an alternate way for OAuth, and what are possible drawbacks of the approach.