The disadvantage of supporting len() is its ambiguousness. Supporting indexing with group name also has disadvantages (benefits already was mentioned above).

1. Indexing with string keys is a part of mapping protocol, and it would be expected that other parts of the protocol if not all are supported (at least len() and iteration), but they are not.

2. If indexing with group names would be supported, it would be expected the support of integer indexes. But this is ambiguous too.

This feature would improve the access to named groups (6 characters less to type for every case and better readability), but may be implementing access via attributes would be even better? mo.groupnamespace().col or mo.ns.col?