Is the Sign In with LinkedIn option worth it?Why are you asking us what YOUR users want?
You can't crowdsource in-house research, especially to the crowd who aren't your users.
If you already have a site that has your own email/password registration, then you should just stick with that. Most users are hesitant to log in with credentials from other sites (particularly facebook), whereas almost everyone is fine with just using an email address to make an account.
Usually a site will use Linked In, Twitter, Facebook or Google login if
The question of a developer or DIY - well just by the way you ask get a programmer - any programmer worth their salt would know how to do this as it needs to be integrated with your own login process.
best of luck