We just use the one offered by Disney. Is that a bad idea?
That kind of depends on what you want covered and why.
The insurance through
DCL only covers those parts of your vacation booked directly through DCL. It does not cover anything you/your TA booked separately -- like flights, pre- or post-cruise hotels, ground transportation, other vacation components, etc.
If you or anyone in your traveling party has pre-existing conditions, the DCL coverage may not be the right fit for you.
Also know that the "insurance" aspect is separate from the "cancel for any reason" CREDIT offered by DCL. That could be an important distinction if you are unlikely to use the credit offered within a year of cancelling (I think it's 12 months, but check that). It will all be lost if not used within the time period specified.
But if you are more than a couple of weeks past booking/deposit, and you decide you want some amount of CFAR coverage, DCL may be your best option. It can be extremely expensive to add later, if even available. DCL coverage can be added up until the PIF date.