card at a time, courtesy of Gatherer's "Random Card" button.
Rivals' Duel (Morningtide, 2008)
This is the very definition of a mediocre card. It's not good by any means, but it's not really bad, either - you can imagine many instances where this card could come in handy. But they'd have to be the kind of specific circumstances that depend on your having built a deck in order to take advantage of certain interactions. Otherwise, yes, it's easy to see how this was a useful card in Lorwyn limited, but it's also worth noting that the Lorwyn block wasn't particularly popular.
Let's unpack all that.
Lorwyn block was built primarily around a strategy called tribal. There are many different kinds of creatures in Magic, and often similar creatures create powerful synergy when played in multiples. Since the beginning of the game there have been decks that focused on Vampires, Elves, Goblins, Merfolk, and any number of other tribes. Because similar creatures have similar benefits and encourage linear play, they can be very effective when played in tandem. For instance: goblins in Magic are traditionally small, cheap creatures who are individually weak and expendable but who gain power in large groups. Elves are also small, but they often produce mana-generating effects which can be used to quickly ramp up to larger creatures or game-ending effects. So while Lorwyn was not the first set to prominently emphasize tribal, it was definitely one of the strongest tribal sets ever created.
And therein lies the problem. While fielding armies of creatures is certainly a part of the game's appeal, it's not the only part. Lorwyn focused on tribal almost to the exclusion of any other theme, and therefore players who did not care for a play environment focused exclusively on tribal interactions found the set to be slim pickings. It's not that Wizards didn't succeed in their goal of creating a tribal-focused set. On the contrary, many of the tribes featured in the set proved popular and powerful: faeries, in particular, became one of the most dominant tribal factions in the game's history, and faery decks still see play in any format that allows them. While nowhere near as popular as faeries, kithkin, ouphes, and scarecrows also had their fans. (OK, I was the guy who liked scarecrows - I tried to put together a scarecrow deck for Commander once.) But the relentless focus on tribal as the block's dominant strategy nevertheless alienated a significant number of players.
Based on that, it's not hard to imagine the circumstances under which this card might be useful. If you were playing Lorwyn limited or standard, environments where most players would be playing creature-heavy decks, having a card specifically designed to hurt creatures controlled by players playing different tribes would come in handy. Say you're playing goblins and your opponent is playing faeries. This card will allow you to destroy one of their guys, providing you have a goblin with greater power than one of their faeries. Or - and this is crucial - you can have two creatures controlled by your opponent fight each other, providing they belong to different tribes. So this card actually supports the tribal theme in two ways: one, it encourages you to play a single tribe and increase the chances of being able to use the card's effect; and two, it discourages you from playing more than one type of creature type, thereby making your creatures vulnerable to this kind of removal. (Also worth noting, if briefly: Lorwyn block also featured a type of damage called Wither that didn't disappear at the end of each turn, therefore allowing for cumulative damage and proverbial "death by a thousand cuts" creature destruction. This card could allow even small creatures with Wither to make an impact against bigger and more powerful creatures.)
With that said, it's easy to see why this card was made in Morningtide, because it fit a very specific purpose and further served to direct players' attentions towards the set's dominant theme. But for that same reason it is of limited applicability outside its home block. At 4 CMC, especially considering it will only be useful sometimes , it's a bit too expensive and too specific to ever be very effective on its own. Because of its limited scope but occasional utility, it would be an interesting card to build around - that is, try to create a deck specifically designed to take advantages of interactions and combos stemming form the card. But again, while it might be possible, that kind of speciality tinkering is a challenge few would find rewarding.