SspId, TermsetId and AnchorId are saved as custom properties within the TaxonomyField. The SPFieldLink allows to override some of the properties of a field when used in a content type (e.g. make a field required based on a content type)
If during the merge process of SPFieldLink and SPField you set SPField.Customization as an element not an attribute of the field, it would be possible to modifiy the Taxonomy custom properties per content type.
Since the Customization Property can be publicly set via powershell, this is a very flexible scenario.
Imagine having a hierarchical termset, where the base parent term to chose values from (AnchorId) could be different per content type.
This would allow for a lot of interesting scenarios.