Is it true that exchange server 2007 can work without edge transport rule and in its absence hub transport role will take all the work of edge transport role

That Sounds good but your Exchange environment is not secure. IF your HUb is internet facing you have to do few things, the filters ( running the antispaagents.ps1 script) , Permissions etc need to modified. Be sure and care ful that your exchange is not open to spoofing /relaying and only authenticated users are allowed to send /receive mail. You need to make your receive connectors are properly configured.

Got the point thanks for the information provided

I am having an effective third party spam software in place in my current exchange environment is it necessary to have hub transport role?????