It appears that what you really want to do is:
S = xlsread(uigetfile);
However, the warning you see is issued by uiimport when doing a synchronous import of spreadsheet data, i.e. one where access to the command line is blocked and the read data is returned as output. Apparently,
data = uiimport('xlsfile')
will not be allowed in future MATLAB versions, as far as I understand the warning.
