It is prudent to place a restoration that protects the biting surface of a
back tooth when the tooth is heavily restored.  If the tooth has had a root
canal in addition to large fillings, it becomes considerably more important,
in the you-better-do-it-or-yer-gonna-lose-the-tooth range.

Leaving the root canal out of the equation, though, most of the back teeth
with large fillings that break can still be restored once they do so.
Usually it's a corner of the tooth that fractures off.

A small percent will fracture vertically and have to be extracted.  My wife
lost a tooth with a little, dinky filling in it this way.  No one would have
looked at it prior to the break and thought it needed a cusp-protecting
restoration.  So, sh--t happens, no?  I'm still trying to get her to do an
implant.

So, the best restoration for a heavily filled back tooth is something that
protects the biting surface.  Crown, 3/4 crown, onlay.  Crowns aren't the
most conservative - sometimes they're the best choice, but they're not the
only choice in many instances.  My brother recently had an onlay after a
minor fracture on a molar following the advice that it would be better than a
refill.  'Course he's well off, can afford the best.  He had a Cerec onlay
which is an all-ceramic, tooth-colored restoration that protects the biting
surfaces.

A lot of folks do not need a cleaning every 6 months.  My patients who fall
in this category are offered a longer interval - usually a year.  Most of
them still prefer every 6 months.

[snip]

Comparison shop.

Go to two or three different dentists (in totally different clinics) and
have each one evaluate you and make recommendations.  In each case,
pretend that you're only seeing that dentist.  Also, don't "lead" the
dentists by suggesting anything - let them all draw their own
conclusions without any prodding from you.  When you're done, sit down
and compare the recommendations from all the dentists; wherever there's
overlap, that's probably something you ought to have done.  Then go back
to whichever dentist you liked the most, and make an appointment to have
the work done.

Some people can go twenty years without a cleaning with no ill effect.
It depends on the individual's physiology, diet, and hygiene.  If you
have gone three years without cleaning, you may be one of the lucky
ones.  Ask them for the specific indications that you need a cleaning.
Then go to another dentist and ask the same question.  In twenty years
you might save $3000 and some time and some cleaning-related damage to
your teeth.