Hi,
It is not mandatory to drop the index before loading the data to the cube. If your daily delta load has more than 10 % of the data(not a hard and fast rule) in the cube then it makes sense to drop the index and recreate it after load. Also if the cube is not compressed then create index time will be more as each request forms a segment and index creation happens on each of these segment. So do the following for your issue.
1) Try loading without dropping index
2) if you get DBIF_RSQL_SQL_ERROR during load which happens due to index not being dropped then go for drop and recreate index
3) compress the cube if you dropping and recreating index.
Regards,
Raghavendra.