It pretty much depends what function taxation serves and what your policy objective is. If it is redistribution of income, then indeed regressive taxation is bad. But if this function is more efficient - or rational, or environmentally friendly if you will - pricing of public goods to change their use patterns, then regressive taxation is ok, probably even better than progressive taxation, because taxes are probably more elastic for the higher income folk, and thus progressive taxes are less likely to produce the intended change in behavior than regressive ones.
Wojtek