From the perspective of someone that runs these type of activities:
Dealing with an unreliable family (and yes she's 10 so it is down to you more than her even if only 60\40) is frustrating.
It's disrespectful to the people running it - even if they're being paid, unless it's their business they're probably not being paid much, and doing it out of live of the activity and to give back to a community you and your dd are part of. Even more true if they're volunteers.
Most have a waiting list so your child could be taking the place of another child more interested and more committed.
Depending on the activity it affects the others attending eg a performance, group certificates/badges, competitions, can slow progress of others if a teacher (dance classes come to mind) has to keep helping someone because their standard isn't as high as others...
Makes it harder to plan things if you don't know who's going to be there.
I also agree its not doing your dd any favours. Everyone needs to learn to be considerate of others, that their lack of commitment can affect others, that even 'fun' things aren't fun all the time (even dream jobs have down points).
I can also see you having problems in the future if she learns that all she has to do to get mum to let her off something is say she's bored/tired.
I have an almost 15 yr old dd myself. She did dance for several years and sadly had to give it up due to HMS. But when she did go "CBA" was not an acceptable reason. They did a performance every term and a big show once a year, it wouldn't have been fair on the teachers or the other students if they were messed up due to dd's flakiness. Swimming lessons I insisted as that's a valuable life skill and we clash if I try to teach her a detailed skill like this. We have free swimming for school age here and she goes regularly (great exercise for the HMS). Brownies she absolutely loved and would have gone daily if she could! When she claimed no interest in this it was always a sign she was coming down with something! Guides the leader was dreadful, everything was indoors sat at tables even in the summer. Heard this not only from dd and friends but a helper who then quit herself. Dd went for a term then quit. Keyboard peaks and troughs, needs chivvied to practice, but feels really good when she does well.
Tired - early bed no screens
Bored - chores
Funny how those excuses soon evaporate!