> Certainly there are things about UFOs which make the time- travel
> hypothesis appealing. Elusiveness, for instance, would seem to be
> a requirement for time travelers.
> However, the variety of reported occupants would indicate several
> races and even species of time travelers, just as ETH proponents
> point to this as an indication of multiple visiting ET species.
> That being the case, I am not sure what we gain by asserting the
> TTH.
Well a few thoughts come to mind.  HG Wells' The Time Machine tells
of a time so far in the future where there are as many off branches
and species of human descended life forms as their are different
kinds of insects on earth today.  It is also possible that these
time travellers ARE human but the result of human adaptions to
alien worlds that humans have colonized.   The idea being that
human evolving on Earth over 12000 years will be significantly
different from the courses of change for people who have been
living on the moon for 12000 years.
It could be that these future humans aren't REALLY different at all.
They simply don't want their great great grandfathers to know they
are coming back for whatever reason and have used technology to
appear as "aliens".   With time travel they could know that in this
time period there was much musing about aliens but they weren't
largely or officially taken seriously.  This might also give
evidence of our time travelling descendants visiting earlier
eras of history as fairies, ghosts, gods, etc.
> 4) Lack of artifacts despite the continued apparent implantation
> of humans with alien technology.
Well I think the easy answer here is that any civilization, human or alien,
advanced enough to travel through space and/or time and implant devices
to gather whatever data that can be accessed acrossed light years of space 
and/or centuries of time must also be advanced enough to HIDE that technology 
from primitive detection methods.
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