In theory, you're right. However, in practice, SaaS pretty much always implies multi-tenancy. Why would one want to host software for someone else who is a single entity?
Many large enterprise customers want complete segmentation between them and other customers. It is not uncommon for critical apps to not actually be multi tenant but a single deployment.
When you talk about consumer apps or low cost business apps your point is well taken.
edit: Sometimes SSO ties it together so single tenants appear to be a multi tenant environment.
Thank you for clarification. Point taken. Nevertheless, I still stand by my opinion, as multi-tenancy is a fundamental SaaS feature for overwhelming majority of deployment scenarios, both B2B and B2B (where multi-tenant B2B SaaS includes most, if not all, of large and very large enterprise solutions, as well, like Salesforce and SAP). What you are talking about in the first paragraph above is essentially a very specific case of SaaS private deployments. I'm talking about a general de facto standard in the industry.
