want to ask u what os the disadvantage using logical datagaurd rather than using active dataguard?
Firstly, you can not use Active Data Guard with logical standby. Only can use physical standby.
As you know, transport service is don't support all datatypes (http://docs.oracle.com/cd/B19306_01/server.102/b14239/create_ls.htm#i76902).
But if you are using logical standby, then you can create additional indexes, also you can use logical database as other database
for other schema because logical standby database opening READ/WRITE mode.