Note that the issue of model release forms and liability waivers is a legal area related to privacy and is separate from copyright. Also, the need for model releases pertains to public use of the photos: i.e., publishing them, commercially or not. The act of taking a photo of someone in a public setting without a model release, or of viewing or non-commercially showing such a photo in private, generally does not create legal exposure, at least in the United States.
The legal issues surrounding model releases are complex and vary by jurisdiction. Although the risk to photographers is virtually nil (so long as proper disclosures of the existence of a release, and its content is made to whoever licenses the photo for publication), the business need for having releases rises substantially if the main source of income from the photographer's work lies within industries that would require them (such as advertising). In short, photo journalists never need to obtain model releases for images they shoot for (or sell to) news or qualified editorial publications.
Photographers who also publish images need releases to protect themselves, but there is a distinction between making an image available for sale (even via a website), which is not considered publication in a form that would require a release, and the use of the same image to promote a product or service in a way that would require a release. Whether or not publishing a photo via the internet requires a release is currently being debated in the 9th Circuit Court of Appeals. It is likely that any and all exposure to the public of unreleased photos via any vehicle will constitute civil liability for the photographer.