It would seem that interfixes are phonemes but not morphemes. This makes more sense to me personally. What is a morpheme if it does not have meaning? Better to call them phonemes and not morphemes. Or to think in terms of allomorphs as @AlexB. suggests.
"A morpheme is the smallest grammatical unit in a language. In other words, it is the smallest meaningful unit of a language." (en.wikipedia.org/wiki/Morpheme) "Interfix, or, more commonly, linking element, is a term in linguistics and more specifically, phonology. It describes a phoneme which is placed in between two morphemes and does not have a semantic meaning." (en.wikipedia.org/wiki/Interfix)
Does it sound awkward when people don't use them in conversation?
I would call it rather a related question as @suizokukan has done.
@MarkBeadles I don't think so, because that question does not involve French, or more specifically, trouver.
It would be interesting to fill in two details. First, how did Norse come to influence the English preposition? Second, was its association with the Latin cum primarily a result of translating the Vulgate into English?
Thanks for the clear and informative answer. Particle of separation is a useful description of *wi. The Sanskrit cognate is also quite cool, combining two familiar parts.