Parrying is very powerful. All non-tower shields have a parry bonus (x1.5 for most shields but there are a couple with more). This bonus applies not just to your follow-up damage but also the amount of damage you can block. So the bronze buckler with it's x2.5 parry bonus can easily carry you through the swamp if you're good at parrying, and you'll enjoy a lot of bonus damage too. Past the bronze stage shields start having lower parry bonuses which might tempt you to not upgrade. HOWEVER, these stronger shields usually have a large number in their block force stat. Block force affects how long enemies remain stunned on a successful parry. So while you might have a lower damage multiplier on the black metal shield than on a bronze buckler, the black metal shield will stun for longer allowing you to make more follow-up attacks. Also, stunning enemies for longer can help with crowd control. Sometimes it's better to leave a stunned enemy and fall back to thin out the minions before re-engaging the tank.