I can safely say that, I started the trend on WTF (ha... I'm a trend-setter...).
When I used to role-play on Gaia, background checks (BGCs) were the norm. It allowed creators to screen for writers who didn't meet their expectations without having to put the individual through some hell in filling out an extensive and lengthy profile.
I started to use the application method (with Halcyon) because a) I didn't want to deal with the traditional profiles for large texts for clarity and b) I wanted to see character ideas before they took full-flight. I don't think it's fair to have people create a full profile and, at the end of it, be told it's not suitable or whatever. I didn't think to do this with Pentimento and, as such, I either had to suck it up and allow for unsuitable characters to continue in the game or flat-out ask the person to change x, y, and z factors. I learned from that mistake, obviously.
I'll assume that other people use the application method for more traditional reasons (i.e. screening players). I use it for a different purpose (although I do still use the BGC for screening purposes, this is just a bonus). If you look to the profiles in Halcyon, they aren't the typical/traditional profiles where people write extensively on a topic. They are short and survey-based.
Typically, the availability question isn't really a huge screener (it wasn't on Gaia, at least) and I don't use it as a way to "disqualify" people. Availability is a general ballpark figure for when you are likely to post. It gives the creator a way to gauge, roughly, when the cast will post as a whole, etc. I use it mainly for the timezone. I need to know what timezones I'm working with so that I can better pace the story/posting.
I would explain a bit more about various other benefits to the issue, but I have to get to class. ):
Edit: Phone. Excuse autocorrects. But, I wouldn't necessarily call this a fad. It's rather isolated and not widespread. It's more of an anomaly, really. (: