Thanks for all the good advices and scary stories, but I am still missing
the actual insight to this matter. The question was not in comparison
between shaper and router. I have a separate router with three different
settings (plunge, D-handle, and the fixed head) for all from-the-top
operations. The question was about the bits and cutters used in the shaper
(http://www.woodstockinternational.com/w1702.aspx). It has three spindles
that can take the shaper cutters with 1/2", 3/4", and 1" round bores. With
the add-on part (router bit spindle), I can use router bits with 1/4" and
1/2" round shafts.

The question was, when to use the shaping cutters and when to use the
routing bits in the same machine. For example, you could cut a 3/4" wide
dado from the side using a cutter head, such as Amana 912
(http://www.amanatool.com/shaper/917.html) or from the bottom using a
straight plunge cutting router bit, such as Amana 45440
(http://www.amanatool.com/bits/45100.html). Obviously the spindle travel,
such as 3", is a limiting factor for some shaping cuts. In some cases, the
shaper table size is not large enough and you have to use the dado blade in
a table saw. The essence of the question is for those cases, where you
could do the same cut either by shaping or routing. For example, there
could be some reasons to use a cutter head for shallow dados and a routing
bit for deep dados. What are the resons for dado cuts? What are the resons
for other cuts?

All comments are highly appreciated, Ollie