As usual Marco's right about everything
They were used to get the rider lower at the front and reduce the drag. For the taller ones amongst us it's not necessary as you can get as low as you ever want to with a 700c front.
You can still source 650c tubulars although it can take a bit of searching.
Other drawback is that on the longer TTs you need to carry two spares as your wheels are different sizes unless you want a very long walk home. You may want to think about that as you've been drafted into the ACC 12-hour team
Thanks for pm - wouldn't mind my bike back whenever's convenient.