Pure shields are useful if you are a defensive battler. The following scenario illustrates it's most effective uses:
1) As a precautionary measure to minimalise or even neutralise any possible attacks that your opponent might use at the beginning of a battle, where most battlers would use that chance to freeze their opponents and either steal or attack at the same time.
This technique gives a chance for you to get a glimpse of what kind of set your opponents has and to respond to those offenses and/or defenses with counter measures using the set that you own.
Best shield for this: Any that you might be equiped with, except
Downsize! (it would be a waste of the D! if your opponent choose not attack at all.
2) During sink. To cover up the gaping vulnerability you have against weapons that deals with earth and air.
Best shield for this: Any shield that blocks air or earth. Substantial amounts of it, that is (eg
Leaf Shield and
Patched Magic hat).
3) During burrow. To cover up the gaping vulnerability you have against weapons that deals with light and dark.
Best shield for this: Any shield that blocks light or dark. Substantial amounts of it, that is (eg
Patched Magic hat and
Mystical White Cloak).
4) Freeze recovery round aka 3rd round. This method is best used when you both find yourself freezing each other, and thus cancelling out each others 'extra' free round. Using it the same purpose as mentioned in the first scenario; gives an opportunity to glimpse at your opponent's set, which by then have no choice but to attack.
Best shield to use:
Downsize!
5) During healing. It is to maximise the amount of healing your pet will be able to receive, while cushioning the impact of your opponent's attack.
Best shield to use:
Downsize!
6) During the last round of the battle, where you are both down to a few health points. You can use a shield to minimise your opponents attack, while you try to hit back with your most powerful weapon. A shield could actually win your battles this way.
Best shield to use:
Downsize!
7) At any round, where you know that your shield will be able to minimalise your opponents combo of weapon, while at the same time dealing a strong enough attack that overpowers the attack of yoru opponents.
Best shield to use:
Downsize! or
Ghostkershield (which is way over stated budget but I am using it as an example).
As to what to replace them with, I will leave it to the advisors to suggest to you. But it is really dependent on the rest of your set and your budget.