It's not particularly daft - if the ride has any hope of opening, they'll have staff there. They won't move them to other rides, which may already be properly staffed, or the staff themselves are not trained on. It gives enough people to ensure that every guest is spoken to who approaches them, and ensures no guests goes into that area. Means that once a ride is available, they have the staff to do their checks and reopen as soon as possible, rather than having to try and find them, ensure that they can be taken off (they could be covering someones break), ensure the break pattern for that ride is correct, that you have appropriate staffing levels for them to run etc.
Presentation wise - there is not much they can do whilst rides down during the day. Clean trains (but that'll be in engineering, so that's a no go). Litterpick queueline (but that'll be done already). Other things they won't be trained on, or have specific depts - like Landscape, Park Presentations (or whatever they call their cleaners). They'll be busy elsewhere.
It might look a bit messy to the outsiders, but it is the easiest, simplest, and best way of going about things. Plus, it's not like the staff cost them a lot anyway.