I'm very sure this functionality is something that is open for debate.  The
current implementation does not meet user expectations.   If you drop and drag a
csv file into Calc it works perfectly.  However if you drop and drag a second
one into Calc, it deletes everything you have already in the sheet in favor of
the new import. In our case users want to be able to merge multiple csv files
that they have exported from other applications into one big sheet.  It's
understood that the users would have to ensure that columns line up correctly on
import.

In Writer, when you drop and drag multiple .txt files, each is put into it's own
section and they are appended to the bottom. This is what users expect to happen
with Calc.