One more check doesn't hurt, does it? Checking at security has nothing to do with what flight you are on. They don't care. They just want you to have a boarding pass which is valid and shows you are authorised to be airside (hence why they don't check ID in a lot of places either). A lot of airlines/airports/ground handlers still only carry out visual checks of boarding cards and don't use digital scanners. In the off chance someone slips through who shouldn't be on that plane then the cabin crew door check is one more safety net.
You'll soon get used to showing your boarding card long before they stop asking for it!
Anyway, I like stepping on a BA plane (for example) and being told "Good morning Mr edi_local, just up/down there on the right/left". I know they don't need to do it, but it is just a nice touch. It doesn't hurt to have your boarding card in your pocket, ready to be shown again.