My guess is that it has something to do with the way ALPS references descriptors.
Basically, in ALPS you would have 
```yaml
descriptors:
   name:
     doc: ....
   create:
      descriptors:
        - href: name
   search:
       descriptors:
         - href: name
```
Which directly translates into
```xml 
<descriptor id='name'><doc></doc></descriptor>
<descriptor id='create'>
    <descriptor href='name' />
</descriptor>
```
While if using yaml capabilities you would have:

```yaml
descriptors:
   name: &name
descriptors:
  _name: 
     <<: *name
```
which after you load yaml using YAML.load would translate into:
```yaml
descriptors:
   name: &name
     doc:
descriptors:
  name:
    doc:
```
which is in my understanding is not the same.
But that is what I remember about this discussion.