I dont think we can set down hard and fast rules for SaaS vendors - each case will have to be taken on it's merit and there will be times when customisation will be preferred and where it will make good business sense.
I think the technical challenge for SaaS vendors is to build a multi-tenanted service platform where customisation/extension is a managed activity rather than an ad-hoc one. Many SaaS vendors already offer per-customer schemas and extension points as part of their service.
The key questions for me are:
Each customisation/extension should be considered using simple product management criteria like - will it cost more to develop/maintain that it will accrue? is there a possibility that the extension could be reused for other customers? etc.
...or to be more precise - who pays?
My shortlist would include SOA, Spring and OSGI.
The capability to be flexible and extensible will be key differentiators for SaaS vendors going forward.