The problem with clinical outlooks and the textbooks is that not all
the teeth read the textbook, so they often don't behave the way we're
taught they're supposed to behave.
    Mostly when dentists talk about failure to replace missing teeth
causing problems they are talking about drifting and moving of the
remaining teeth.  This does happen, but not uniformly.  If my patients
truly prefer to defer replacing teeth, I am happy to monitor for
problems over a period of time.  Sometimes it is a good idea to make
models of the teeth for ease in assessing if and how much movement has
occured.
    No one should be forced into treatment they don't want; and so long as
the patient is fully informed of what may happen and is followed
regularly I see no problem; you always have the option of changing your
mind down the road, so long as not too much movement has occured.

Steve

If your masticatory ability hasn't been affected a lot by the missing
teeth then you should be ok from a functional point of view without any
restoration. Moving and drifting teeth can be a problem, but not in all
cases. Generally, if your occlusion is relatively stable (each tooth
contacing two opposing teeth), teeth should not move. The dentists at
the US army did a very large study over drifting teeth after teeth
loss. They found that any tooth movement will occur within 5 years or
not at all (unless conditions change of course).

As for cleaning, you could say bridges are more difficult to clean
because they require you to use more tools than the traditional brush &
floss. IMHO, superfloss is probably the best way to clean under a
bridge. Bridges are more vulnerable than natural teeth and thus require
a higher level of oral hygiene to keep in good condition. Once you get
used to an effective daily routine, you probably won't notice the extra
effort you're putting in.