Lots of problems with this:
1. excess baggage insurance is specific to each carrier; if you connect to a different carrier, you are not covered unless you claim the bag and recheck it, paying a new excess baggage insurance charge.
2. most insurance covers loss (only) and not damage.
3. many items are excluded, such as computers, jewelry, etc. Almost certain that no airline will insure a musical instrument under any circumstances.
4. maximum varies by airline, but on AA the max is USD 5000 and cost is USD 2 per $100 of coverage.

Most airlines have similar rules. In short, very expensive insurance to cover very little -- compared to the other alternatives available. Its only merit is anyone can buy it when checking in (the alternatives require preparation in advance).

That said, I've shipped computers worth >100,000 by suitcase and proper customs documentation, with no problems, so it is possible, but it does make for a high profile target plus baggage systems have some rough handling. Some airports are pretty dangerous for this sort of thing (LHR and AMS spring to mind, while HKG is pretty safe).