Well, thanks for contacting me :-)
My use case is that I have several mailing accounts inside Thunderbird, two of them being extra secured: My business account and my really personal account (I have some that are slightly in between the two: These are "disposable", meaning, when they receive increasing numbers of spam, I will just deactivate them).
My business account is not used for "run of the mill" contacts, it's strictly business, with partners I know and trust, and they trust me. My personal account is the same, just not business. If these accounts would see identity theft it would really hurt me. That is why I secured them with passphrases more than a decade ago. Thunderbird will cryptographically sign all e-mail that go out from those, and therefore Enigmail always prompted for the passphrase when sending out a mail (with a grace period of a couple of minutes in which passphrases will not be asked for a second time).
I do not secure Thunderbird (or Mozilla) with a master password, because I find it enervating to have to enter this every time I start up mail or browser. I do have my OS account (on Linux Mint) secured by a password, but usually I enter that only once a day. When I have guests, they are free to use mail or browser.
I don't mind when they use one of the disposable addresses, but they can't use the accounts that are secured - because they don't know the passphrases.
This personal scenario with friends in my point of view would be applicable also to families. I know no family where family members would log in and out on OS level to strictly separate data and accounts, as a business would certainly do.
This leads to all family members using one family mail client or browser! But it would certainly not be desirable if the kids would be enabled to send out certified mails (!) bearing the digital signature of the parents... In such a scenario, were a family shares the OS account but has separated mail accounts, passphrases would be the method of choice to secure the digital identity of those who wish to do so without making the bothersome lock-up of the PC with personal accounts for everyone with constant logging in and out...
Another point I would like to raise is that this grave difference in behaviour between Thunderbird and Enigmail comes undocumented. Users importing their Open PGP keys are not being warned that the two times they have to enter the passphrases for the import will forever be the last times those passphrases will get verified!
I do see that for some users it might be desirable not having to enter passphrases. This leads me to the feature request that passphrase verification be made an option (with default on). Let the users decide. But if someone created a passphrase, I believe it is unlikely that such a user doesn't care about that passphrase in the future...