I suspect the team sponsors enjoy having the team prize - gives them some nice photos to use in motivational posters etc. I don't remember seeing the yellow dorsals until a few years ago though.

Your suspicions on verifying the winners though, are ill placed:

http://en.wikipedia.org/wiki/Team_cl...Tour_de_France

Year Team
1930 France
1931 Belgium
1932 Italy
1933 France
1934 France
1935 Belgium
1936 Belgium
1937 France
1938 Belgium
1939 Belgium B[note 1]
1947 Italy
1948 Belgium A[note 1]
1949 Italy A[note 1]
1950 Belgium A[note 1]
1951 France
1952 Italy
1953 Netherlands
1954 Switzerland
1955 France
1956 Belgium
1957 France
1958 Belgium
1959 Belgium
1960 France
1961 France
1962 St. Raphaël
1963 St. Raphaël
1964 Pelforth
1965 Kas
1966 Kas
1967 France
1968 Spain
1969 Faema
1970 Salvarani
1971 Bic
1972 Gan – Mercier

1973 Bic
1974 Kas
1975 Gan – Mercier
1976 Kas
1977 TI-Raleigh
1978 Miko – Mercier
1979 Renault
1980 Miko – Mercier
1981 Peugeot
1982 Coöp – Mercier
1983 TI-Raleigh
1984 Renault
1985 La Vie Claire
1986 La Vie Claire
1987 Système ‘U’
1988 PDM
1989 PDM
1990 Z-Peugeot
1991 Banesto
1992 Carrera Jeans-Vagabond
1993 Carrera Jeans-Tassoni
1994 Festina
1995 ONCE
1996 Festina
1997 Team Telekom
1998 Cofidis
1999 Banesto
2000 Kelme
2001 Kelme
2002 ONCE
2003 Team CSC
2004 T-Mobile Team
2005 T-Mobile Team
2006 T-Mobile Team
2007 Discovery Channel
2008 Team CSC Saxo Bank
2009 Astana
2010 Team RadioShack
2011 Garmin-Cervélo