8.1.6.0.0,nt 4.0

We are running hot backups .what happens if we do not backup contents of all the tablespaces i.e we do not run following for, let us say, 2 out of 7 ts:
alter ts ts_1 begin backup
copy df using host command
alter ts ts_1 end backup;

how will it affect the recovery when it becomes necessary?