When I was in school we in fact did NOT use bonding techniques with
composite.  Bonding was discovered in the mid 1950s, but I don't know if
the early studies used unfilled resins as adhesives.  Bonding can be
done without a bonding agent, but it's much less effective.
    Naturally, resin fillings done without bonding were a pretty dicey
proposition because of the amount of dimensional shrinkage involved.
But since these were mostly used at that time only on small lesions on
front teeth, the potential damage was minimized.  Composite was still a
big improvement in some ways over what was used before--acrylic resins
and silicate cements.
    Amalgam does not adhere to the tooth, but it does expand on setting.
The dimensional change is much less than that of even the best composite
resins, but it cannot be bonded effectively (the Parkell company on Long
Island was claiming for a while that their product "amalgambond" could
bond amalgam, but I have my doubts about that--I can't even conceive of
how that would work.
    Amalgam does leak--esp. initially, until it tarnishes.  In the old days
we would line the cavity with copal varnish to seal the margins until
then, but in the 1970s some copper was included in the alloy mixes know
as "dispersed phase" alloys.  These had improved marginal qualities,
esp. over time, and we stopped using varnishes.
    All restoratives leak--those that set in the mouth (composites,
amalgams etc. may have setting expansion or contraction), and even those
that are formed outside of the mouth may leak as a result of differences
in coefficients of thermal expansion between the restorative and tooth
structure.  Probably the best restorative in this regard is condensed
gold, but these are very labor intensive and time consuming to place,
and are seldom seen in the world outside of dental licensure exams.

Steve

Amalgams don't actually ADHERE to teeth. They are kept in place by the
shape of the tooth.

In the process of preparing an amalgam restoration, the dentist
removes the decay and shapes the tooth so the amalgam won't fall out.

The amalgam is the consistency of modeling clay when it is placed in
the tooth -- it can be shaped and packed into the retentive recesses
that the dentist has carefully formed in the tooth structure. In a few
minutes the amalgam hardens, and then cannot fall out.

Think of an empty wine bottle -- the inside of the bottle is larger
than the narrow neck opening. You can stuff modeling clay into the
bottle, but after the clay hardens, you can't get it out. The neck
opening is too narrow.

That's basically the same principle that holds amalgam in place. It is
shape alone, as no adhesive is generally applied.

Today, most fillings are made of bonded composite resin, and the
retentive grooves needed for amalgam are not needed for composite, so
the shaping of the cavity by the dentist is somewhat different.

Some day, the shaping of the cavity to receive amalgam may become a
lost art, much the same way that the shaping of the cavity to receive
old-fashioned gold foil has almost become a lost art -- kept alive
only by a few dentists who regard it almost as a hobby.

- dentaldoc