Here are some examples of funeral customs and weather flowers are or are not used.
http://www.nsm.org/guides/religions.html
Cultural funeral customs. Good read. Most comment that burial customs are done to comfort the family and bereaved. Many cultures don’t use flowers. Some employ cremation under the belief that it frees the spirit.
http://www.eulogywriters.com/funeral_customs.htm
This one suggests that modern western burial customs originated in Egypt. And that similarly, a variant of burial customs largely represents the class of individual. It nicely describes the growth of the funeral ceremony.
http://members.aol.com/taimission/funeral.htm
The one above is about Taiwanese Funeral Rites. Also a good read.
http://www.morbidoutlook.com/nonfiction/articles/2003_06_america.html
Lastly, the one above covers funerary practices in early and contemporary America.
It is interesting that with the growth of civilization, the funerary rituals tend to move away from the home and have become yet another institution, sadly complete with rituals that seem to benefit capitalism more than anything else.
There was one reference I came across but didn’t save the link, where a modern anti-institutional even retro-trend of funeral burial is to perform burials in wooded areas as opposed to a cemetery.
It would seem that role of flowers in our burial culture has a direct relationship to the importance of ceremony for the family of the bereaved.
On the other hand, as someone above said, modern funerals are mostly for the living. It is very painful to loose a loved one. And a lot of the element of ceremony is about the important tribute of showing sympathy. While some suggest that fear of death are what funeral ceremonies represent, the importance of sharing a kind moment by gesture or symbol with someone at a time of grief should never be considered unimportant.