Obviously it's always pretty much about situation, right? If you play against rushdown, these cards are next to worthless. If you play against some big late game stuff, they'll probably be amongst the most efficient to have.
These are, of course, the extreme cases. These cases are worth considering for the meta aspect of things if nothing else. If there are lots and lots of rushdown decks currently being played, these cards are less "relevant" so to speak (I guess this is sort of the approach you wanted us to take when you asked us to discuss relevance?). So the relevance of the card is dependent on what type of deck you face. Not surprisingly, the relevance of these cards and indeed the relevance of any card is dependent on the type of deck you put them in as well. If you're buiding a deck and have very few to no ways of dealing with heavy minions in that deck right now, you'll want to consider these cards if you shoot for a balanced deck.
Now, if we are to set a general value on these kind of removal cards we'll want to consider how likely they are to be efficient, right? In what cases do they pay off and when are they more of a burden to have in your hand instead of some other card? So we already know the extremes, but what about having them against a balanced deck that has minions as well as spells ranging from low to high cost? Turns out that generally speaking, you are likely to get a good tradeoff against these decks. This is why these types of cards are rated high overall and are in alot of decks: their purpose, to remove big minions, is pretty general and as such pretty good. There are big minions in alot of decks and even if you're unlikely to hit Ragnaros or Ysera, you are likely to hit something worthwhile. In some situations, polymorphing a Chillwind Yeti can be worth it. If someone uses Blessing of Kings on a minion these cards practically gets rid of both the minion and the Blessing of Kings, which means you've traded one of your cards for two of your opponents, leaving you with a card advantage.
This wide range of efficient usability is what makes these types of cards very relevant to consider in most decks.