Interesting, I don't recall seeing any Planar 6, 7, 8, 9, 10 selling without arm.
No. Rega decks are almost always sold as units, except the cheaper decks which are often split, but I don't know that it's such a terrible idea. If I sold the RP10 for say £1000 it lets someone get a top spec deck at an affordable price and they could use any Rega arm on it for the time being, upgrading the arm later.
If you think about it, people do that with LP12s all the time. Buy an older or Magic LP12 and upgrade it as funds allow. Theoretically, you could do that with Rega decks too.
Rega clearly don't want you to though. They have never sold a turntable without an arm, or even with a choice of arms. Linn have done that from day one.