I feel that we should not generalize the fact that SaaS business in the cloud must be only multi tenant. Effectively a company like SalesForce needs to be multi tenant. Should company providing HR solution to enterprise also think the same? Yes after they address company providing HR solution needs to keep end customer in mind and be ready for single tenant model too. The challenges with Cloud + SaaS makes me think so.
1. For company offering SaaS, being dependent on other PaaS/IaaS vendors is a newer concept and creates complexities and is uncharted territory and meeting SLAs together is evolving and generares competition and conflict. In addition, building multitenancy into the product proves to be challenging and expensive.
2. When you continuous upgrades and support, and will always benefit from the highest point in the evolution cycle of the product, the product is going to change over time, and the customer must be willing to go with the changes. This is not true with large enterprises. I have seen SaaS vendors offering free supporting older version as users do not want to mvoe to the newer model
3.if customers want customization specific to them, yesterday they did not have options, today they have the cloud.
4. There are no standard security mechanisms where multiple vendors provide authentication using a single authentication user name and password for multiple SaaS services and forces end user like me to remember multiple passwords.
If you have vision to beome some one like SalesForce, they need to think of multitenancy in their model. if they have vision to handle enterprise, then they need to think of both multitenancy and single tenant model of deployment. I would expect that SaaS providers need to think of both multi tenanct and single tenant model.