A flat tax would be much more equitable (and fair) as opposed to just blindly taking money from wealthy people because "they won't miss it that much".
A flat tax is fine if it's enforced (no exceptions except for the the really poor).
AND: Regressive taxes and fees that effect lower income people more heavily are reduced.
For example, sales tax:
Two people buy $1,000 on household goods for their home and pay $20 in taxes (5%). The person making $20K/year has eaten up 0.1% of their income on taxes while the person making $200K/year has lost 0.01%.
I'm all for a flat (progressive) tax if we can get rid of the regressive tax structures.