According to Jacques Vallée, the extraterrestrial hypothesis was the predominant explanation for UFOs decades ago.Wikipedia
The interdimensional hypothesis, is an idea advanced by Ufologists such as Jacques Vallée that says UFOs and related events involve visitations from other "realities" or "dimensions" that coexist separately alongside our own. It is not necessarily an alternative to the extraterrestrial hypothesis seeing as the two are not mutually exclusive so both could be true simultaneously.
It seems to me that this picture has changed significantly with much more usage of the term interdimensional but ET has become such a clichéd legacy that I know I use it when in actual fact I mean interdimensional.
I'd be interested to hear in what circumstances or cited sources you use the term ET and mean specifically that which the term references?