There is a space for a discussion if/what cards could/should be included as baseline upgrade-research options, but overall outside of that I don’t think there is such a thing as a useless card. Unless it’s designed to be. But even a card that gives some utility or cosmetic options is valuable for someone in some way. Most people don’t play (comp)multiplayer or MP at all, and when playing solo or coop the term ‘value’ can have a rather wide meaning.
When it comes to cards mentioned by OP- I disagree with both examples. I mean they could be included as research. But that goes for the majority of baseline cards, like upgrades to productivity, mercenaries, training times, watchtower limit, or factories. And the other way around for some things.
The game is designed around (fantastic!) shipments-civ customization mechanics and picking and choosing cards to assemble a set is a skill requiring knowledge, plan, and is a joy in itself.
I never felt like boosting explorer’s HP and giving him an animal companion is crucial in any way. Especially as the time went on and different hero classes were introduced. Some civs don’t have a card like that, for many balance reasons. Just like not providing that upgrade for free or without a card.
With advancements in age he gets stronger and can take bigger and bigger treasure guardians on his own.
Using an explorer card is a decision. The decision be more aggro in play and exploration and sacrifice important card that would be used instead like free vills, resources, and gathering rates.
That is not a universal decision, and the game plan varies between civ picks, maps, modes. Making it always available makes the game slightly more shallow. For the cost of what, making it slightly more streamlined.
When it comes to balloon it might be more suitable, at least for few European civs, but most of the points still stand. It’s an investment into field recoinessanse and I don’t see ballon to be so important and irreplaceable and even mandatory, that it should be a normal upgrade.
You can just send a scout, cheapest horseman or trash native to scout the map and it works fine. It’s not the same and the balloon can be super powerful and match-saving/changing but that is another argument for why it should be gated within player shipment inventory.
As the match goes on its value grows, and searchable cards don’t really grow in cost. They can, or every age can offer a ‘different’ ballon that can be trained for different cost, but that’s just making things more complicated and bloated. Time for these changes was maybe in 2006 or 7, or around the release of DE. Same for the purpose and usability of priests, medics, spies and few other units or mechanics.