Good points. There has been research into the idea that comets could have warm centers due to radioactive heating, sufficient to allow liquid water. Some concluded therefore that life could persist within them. If so, then comets could provide the means of panspermia between star systems. I suppose the same could be said about asteroids.
See some of the references here:

The Net Advance of Physics: ANNOTATED BIBLIOGRAPHIES, No. 1
Panspermia Theories: Annotated Bibliography, Section V
ORGANIC GRAINS, PREBIOTIC CHEMISTRY, AND LIFE ON COMETS:
http://web.mit.edu/afs/athena.mit.edu/user/r/e/redingtn/www/netadv/bioast/clash/comets.html

M. Wallis, 1980 Nature 284:431.
Argues that the center of a comet would be originally liquid, due to
Aluminum-26, and potentially hospitable to life.

A. BarNun, et al, 1981 Origins of Life 11:387.
Attack the idea of Hoyle and Wickramasinghe that life arose in
comets. Most organics are sublimed or blown off at perihelion in too
short a time to migrate into interior; the structure is too tenuous
for a liquid core; if there were a liquid core (due to Aluminium-26),
it would be too radioactive for life.

E.K. Jessberger and J. Kissel, 1987 Lunar and Planetary Science
Conference XVII:466.
Report finding no evidence of excess decay products of Aluminium-26.

I.D. Hutcheon and R. Hutchison, 1989 Nature 377:328.
Present evidence of excess decay products of Aluminum-26 in
carbonaceous chondrites, implying this isotope may have been abundant
enough in the early solar system to cause major heating of small
bodies [e.g., the liquid comet cores of certain exobiological
theories].

S.S. Russell et al., 1996 Science 273:757
Report excess Aluminium-26 decay products in ordinary chondrites and
argue that this reflects high Al-26 level in Solar Nebula.

Bob Clark