Looking at how empty the line for that side is for most rides for most of the day on every visit I've made in the last year with an AP, while considering typical attendance and even maximum capacity in these two parks are, it would be difficult for me to imagine that even remotely close to half of them are a la carte.
It's 2,400 rooms that have access from the three hotels that are eligible. At a conservative 82% occupancy, that would be 1968 rooms. Lets say 2.5 people per room which is also low since standard rooms sleep up to 5 and almost 100 rooms are suites.
But going with what are likely low average numbers, that works out to 4,920 people.
The two parks combined have a maximum capacity of about 56,000 people which on a
typical day, they never come anywhere close to hitting.
If you're a Harry Potter fan and spend a big chunk of time in those two areas, you might get the wrong impression about how many people are in the two parks, though - especially Diagon Alley.
With all that in mind, I'd find it completely believable that Express Pass usage hovers around 12-15% of total guests on most days before 4pm.*
I can't see how even remotely close to 5k people are paying for it on a typical day which is roughly what you'd need for a low estimate to call them the majority.
Frankly, I would have a hard time imagining half that doing it, most days.
The only time I ever see it start to pick up (and also when wait times tick up a bit) is in the late afternoons which is when the top tier AP holders are allowed to use it, too.
Of course, I don't go during winter break and times like this week. I'm sure the a la carte is a
lot higher at the moment - maybe even as high as 30% but of course, rooms are closer to 100% capacity with more people per room, too.
That said, while value is entirely subjective, I can't see any reason for someone to pay for it outside of a handful of holiday weeks unless they can't spend a complete day because the only ride we've seen with a consistently long wait is Hagrads. Most have posted wait times of 45 minutes or less for most big chunks of the day.** Often, everything hits somewhere between 10-30 minutes posted wait time at
some point.
... and almost without exception, those posted wait times are longer than the actual wait times.
I swear, a posted wait for Forbidden Journey of 20 minutes is just their estimate of how long it'll take you to walk that massive queue that they can't shorten to get to the loading platform.
Apples and... staplers are also both things that exist and are sold. I wouldn't see the point in trying to compare them either but have at it - we'll have to agree to disagree.
*They're typically low usage is why they're able to do it without reservations and allow unlimited re-rides for the resort guests and top tier AP holders who are allowed to use it every day after 4pm.
**Velosicoaster and Gringots are two possible exceptions but both - especially Gringots also have periods with shorter waits - you just have to watch more for them. We did Gringots back-to-back 3 times around 1pm on our last visit on a Saturday about a month ago followed up by the mummy 2 times immediately after)
