Mark,

This feature would be great. It was one of the comments also yesterday evening I got in my demonstration of Composer.
When/Why this feature.

Those who download Midifiles, process these into Music notation and rehearse do not really mind this feature.
The serious music composers, traditionally skilled to write compositions, as well as Dirigents that make arrangements will strongly require such feature to add all kind of music notations like "Ritardo, FFF, PPP" etc
For a part you come around it with free text, but your competion built in such library in middle class programs. You better have it to be not negative quoted here.

Jaap