The question becomes, as an author, should you number the books in your series or give them stand out titles and subtitles? Or both?
I'm going to come out and say it, if you simply number your books in a series as Book 1, Book 2, Book 3, then chances are nobody will remember your books. They might remember the main title, but the reason subtitles stand out is that humans tend to remember things via what is called associative memory.
It's because our brain makes links, or associations, between characters and events in a narrative that we can better remember a title. In fact, mnemonics or "memory theater" is one way we can learn large amounts of content, either using acronyms or else developing little narratives to aid in information retention. When the narrative is already there though, the information is easier to recall. Which is why some people are so excellent at quoting books and movie lines. It's also how we can recall the name of a character or title of a book.
We know, for example A New Hope if where a holographic Princess Leia pleads, "Obi Wan, you're my only hope!"
Everyone knows the name Obi Wan, because this line is so ubiquitous with Star Wars, even if they haven't themselves seen the film.
We know that in The Empire Strikes Back this same Princess calls Han Solo a "scruffy looking nerf herder," and when Chewbacka laughs Han tells him, "Laugh it up, Fuzzball!" When Luke goes to be trained by Master Yoda we remember Yoda's sagely words, "Do, or do not. There is no try." And upon Cloud City, Luke faces the Lord Sith Darth Vader, who reveals to Luke the startling revelation that, "No, I am your father."
In Return of the Jedi we remember speeder bike chases through the forest of Endor and fluffy Ewoks and Jaba the Hutt. All of these things are association we make between the events of the film / story and the title of that film / story.
If it was just a number, we would have less incentive to remember what set each movie apart from the rest.
Which is okay, if you want your series to all seem like one epic story, like Lord of the Rings.
But if you want your books to stand out as original chapters of a much larger epic, then I recommend using original subtitles over simple numerical sequencing. Also, numbers are boring. Personally, I think if you only number your books then you lack a certain amount of imagination, because otherwise you would of thought up an original title, right? If an author can't bother to come up with an alluring title and subtitle, then I have to wonder if the rest of the book will be all that good as well.
Also, numbering does make me get excited. Bitten: Resurrection, my zombie book of The Resurrection Virus series, says all it needs to. The second book, Bitten 2: Land of the Rising Dead contains both a numerical heading and an original subtitle.
I chose to use both because Bitten 2 involves an entirely new cast of characters, in a different country, but I wanted readers to know it's still part of the same Bitten universe. Likewise, the upcoming Bitten 3 is titled Kingdom of the Living Dead. The fourth Bitten novel will be Bitten 4: Ancient of Dread Days. Currently I am plotting Bitten 5, but it's tentative title will be Bitten 5: Ragnarok. A nice end-cap title to compliment the first title: Resurrection.
With my Scarecrow & Lady Kingston paranormal investigation detective series, I have opted to leave off the numbering entirely and just stick with original titles. My first book in the series is called The Scarecrow & Lady Kingston: Rough Justice. The second book will be titled The Scarecrow & Lady Kingston: A Slice of Grilled Americana. Although the books are part of a series, each story stands alone on its own, so there is really no reason to link them numerically.
So the question is, what is right for you?
Do you want your books to stand out? If so, then original titles and subtitles matter.
If you want people thinking it's one giant epic, and you want them to view each book as a chapter in that epic, then perhaps numerical headings after the series name will suit your needs.
Of course, you could always choose to use both, as I did with my Bitten books. Really, it's up to you. But if it were up to me, I'd always opt for more original titles over ordinary, boring numbers.