The main reason for having multiple keyspaces is because some settings are per keyspace, whereas some are per column family.
The most important per keyspace settings are for replication - replication strategy (single or multi DC) and replication factor (overall or per DC). All column families within a keyspace have the same replication settings. Different applications may have different replication requirements - maybe not today but may in the future. So for this reason it is a good idea to use different keyspaces for different applications.
In terms of performance or resource usage, it makes very little difference whether column families are in the same keyspace.