Well said, Chuck! Technically, using the dictionary definition, hybridization covers species, subspecies, colour phases, cultivars, etc. as a general term. So yes, I'll agree that with this definition hybridization happens rampantly in nature. It could even be argued that any form of sexual reproduction (to parents with different genes, though only slightly different) could be viewed that way, if you took the definition strictly enough. However, from a biological point of view these things are quite different, and we generally differentiate between them with additional terminology.
'Intergrades' generally do "merge into each other in a series of stages, forms, or types" in the wild. For example, in North American ratsnakes, moving northwards, Everglades, Everglades X Yellows, Yellows, Yellow X Blacks, Blacks (yes, there are others as well). In fact, as it turns out, these various forms may all be more closely related together genetically despite their outward appearances than Blacks from one side of the Appalachians are to Blacks from the other side.