As long as everybody is on your address book it's fine, but if they're not, things tend to get awry.
I haven't tried the new Thunderbird, but I am suspicious of this change as well.
This was my first reaction when I saw the change - I really hope they get this right as it goes to the core functionality of Thunderbird. If email addresses can't be easily and accurately edited in the To field, it will be a real problem for me.
I've used every major email platform and every single one manages to endure despite having emails on one line. Even Outlook does one line these days.
Its just yet another thing I shouldn't have to think about