To the "mushroom fans" I'd add that the literary analysis of apocalypses in general and Revelation in particular points to a very conscious intellectual process of composition combining textual allusions and references with a particular "world vision" including political and religious ideology. The "fantastic" effect is due, in part, to our reading those texts from a completely different cultural perspective, in part to the real creative quality of the works.
Terry's question (why symbols instead of plain expression) is interesting. Hiding the meaning from real or potential persecutors is a common answer, but it is not always relevant: not all apocalypses belong to a persecution context; the meaning is clear anyway from the included interpretations (no ancient reader could miss the description of Rome in Revelation 17--18). Another obvious motive is to vest the message with divine authority: the writer first receives a supernatural vision which he doesn't understand, then a supernatural messenger explains it (cf. the role of trance and interpretation in Greek prophecy). But more deeply, this raises the question of the use of non-verbal symbols along with language in communication in general. By using them, and constantly shifting from the visible to the intelligible and the other way around, we mean an excess -- whether this is construed as an excess of meaning over "mere words" or an excess of reality (even "imaginary" reality) over meaning. Paradoxically, a part of communication consists in pointing to what is lost in, or left out of, verbal communication.