If you have read some of what I have written about in the past I really hate when games put in mechanics that masquerade as doing something more. Games that put in a mechanic to give an artificial sense of difficulty, choice, etc. To me, in many ways, that is the case with collectibles in games. In essence, collectibles create an artificial sense of exploration and pad gameplay time - and in most instances collectibles are implemented lazily.
To me there are only seems to be a few purposes to collectibles (that developers have given them) these days: 1) to pad content time, 2) fodder for achievements, and 3) create artificial exploration.
For many games, getting all of the collectibles takes a significant chunk of time. And to what end , satisfaction, or value is that padded time? Having to collect 100 of one thing never really seems to add anything to the vast majority of games other than to say "yay I did it," which then usually comes with an achievement.
The way that collectibles are handled now is a disservice to those gamers that enjoy gathering achievements and completing games to 100%. As of right now collectibles, for the most part, are nothing more than a chore. Often, it boils down to putting every inch of a map, level, area, etc. to intense scrutiny to find that one little thing, whatever it is.
I guess, technically, that is sort of exploration as looking for the collectibles does make you look around maps some more, but its it good game design? What is the value to that exploration other than to reach that collectible? It is just lazy.
There usually isn't even something as simple as a neat little scene of some worth. There is nothing to reward going after collectibles, other than an achievement usually. When you do the work to find something (or just look at a guide), the moment you collect it you might be met with something like "57/92," or something similar.
At least with games like Shadow of Mordor there is some lore attached to what you are collecting that help develop the context and history of the setting. Something like that is simple enough to satisfy most, but it still doesn't address the fundamental flaw with collectibles, which is how you go about collecting them. The reward for collecting is just the other, smaller, half.
Too often now collectibles have been reduced to just something on a map or minimap to just go and get, which is exactly what Shadow of Mordor did. There doesn't seem to be a point to something that should essentially have its foundations in exploration/scavenging when you know exactly where it is before you go looking for them. At least Shadow of Mordor, or something like Assassin's Creed: Brotherhood with the Assassin Tombs, soften the blow with what you are rewarded.
It doesn't help that for many games now the existence of collectibles almost seems to be an obligation, which is probably why there are so many thrown together lazily. Ironically, they are implemented poorly and likely due to that obligation, but overdo it in what they include as there are often well over 100 things to collect in many games.
This is a case where less is more is really good advice. All the time it probably took just to find places for all of those collectibles could have been better spent reducing the number significantly and figuring out some cool way to implement said collectibles.
Assassin's Creed IV: Black Flag had a pretty cool way they did some collectibles with the treasure maps with some drawings that told you where to go generally but then it was up to you to figure out where the treasure was. Something as simple as that, which plays into the setting of the game, is a good example of what developers should strive for.
But, seeing as most developers will likely just keep the status quo obligation of lazy implementation, I would just much rather not see collectibles at all.
Like I said before, if I was one of the gamers that love to get games completed 100%, I would be extremely annoyed at all the time necessary to find all the stupidly hidden collectibles around a game. Collectibles shouldn't be there just for the sake of being there.
Give them a purpose, reward not only the effort of collecting (like reading some lore), but reward the process of finding the collectibles in some way. Right now collectibles are mindless and meaningless, but could become a cool addition to a game if done in an interesting game. Developers should take advantage of something that can add something to a game and is relatively simple to implement - and there doesn't need to be near 100 things to collect either.
In other words, there is a good opportunity for collectibles to become a cool aside to the main part of many games that most developers have ignored so far. But, as long as they choose to ignore that opportunity, I would much rather just do away with collectibles altogether.