hi Liz,
i think you're right about it being a case of perseverence, you can wean off them. You could try starting the feed with them on and then breaking the latch, whipping the shield off and sticking lo back on all v quickly (easier said than done!) this is what i did with dd1 and it did seem to work eventually. They do prefer the shield as its bigger in their mouth.
it is better to not use them cos apparently they can affect your milk supply, however i have known a couple of ladies feed for 7 months exclusively using them so i wouldnt get to stressed about it if lo wont feed without them. hth x