Just to follow up…
As luck would have it, I had to make another trip to Paris for a meeting yesterday, and I see that my account has been upgraded to Carte Blanche this morning ^
I think what I will appreciate the most is the ability to use the Business Premier ticket office and check-in line. For a number of reasons outside of my control (last minute request to attend a meeting, which came while I was out at lunch), I arrived at St. Pancras without a ticket, about 50 minutes before departure of the train I needed to take. It took 26 minutes of waiting in the ticket queue before I could speak to someone, and by the time I had my ticket, it was 19 minutes to departure. After waiting in the security and immigration queues, I boarded the train about five minutes before departure. It was quite stressful as I did not know if I would make the train, and therefore, I did not know if I would be able to attend the meeting. In retrospect, I realise this was the second or third time I have found myself in such a situation (and I emphasise that when I travel on Eurostar for personal reasons, I always arrive well in advance and am never late)!
So, for me, I think that the ability to use the Business Premier ticket desk and check-in queue is almost enough benefit, by itself, to warrant the extra effort for Carte Blanche (at least, for those of us whose business will not pay for Business Premier). It seems silly that one must wait for all of the tourists trying to change unchangeable tickets when all one wants to do is buy a ticket at the last minute price and go. Anyway, I'm very glad that I will not have to wait in the normal ticket queue again!
This makes me wonder: Why do Eurostar not offer ticket machines? I understand that they offer machines to collect previously-booked tickets, but why not enable the kiosk to sell tickets also?
Again, thank you for your advice! Must change my FT profile now…