I'm not sure if Google+ and Spotify are the best cases for whether or not the invite system created value--they were both already benefiting from a ton of coverage and widespread awareness before they launched.
Personally, for the website I'm currently working on I plan to implement some form of the "viral invite system". I see it as being beneficial in two primary ways:
One, it can help me with scaling to match the userbase should I choose to lock new user registration. I'm not expecting an explosion of users to come rushing to try my website/application, but with any growth unforeseen problems will arise and this can help me deal with them.
Two, the hype you mentioned: by giving earlier access to users who share your website out, you can help generate some good word of mouth. There's already a hurdle you have to cross to get users to give up their email address to you, so it's not much of a stretch to encourage an impulse action--getting them to share your link out across Facebook/Twitter.