What scottNUMBERS said.

You are dealing damage, just not as much of it because you have to choose high def or high attack.

Don't forget that a sub job is only able to max out at half or your main. So if you sub a Smn for example to your lvl 50 Whm, your summons will only be operating at lvl 25.

This also applies to Job Abilities. Say at lvl 5 a war gets provoke (not sure on the actual lvl) but your Pld is only lvl 9. You won't have provoke until lvl 10 Pld making your sub max at lvl 5