your wrong m8. i cant see is celica being sprayed in cellulose paint and 2 pack needs laquer (clear coat) for protection from stone chips, scratches, etc... not to mention the shine.
basically a base coat is made of a colour coat pigments, mettalic pigments or pearl-effect pigment then a bonding agent is added then the solvent to make it liquid. if cars rolled out of the factory without protection then you would be fetching your car back within a week ownership due to oxidisation and chips. you would be investing in shares with t-cut. like you said you can paint a car with just basecoat but it wouldnt last. cellulose is different though as you have cut & compund the paint then every 5/6 weeks wax it or it will dull but not having to worry about the clear coat.