There are certain discussion loops and services available only to Pro members, and there's a Pro retreat at National, which I think is just a series of workshops on the first day of the conference. (I'm going to National for the first time this year.)

Anyway, I am RWA-Pro, but it never would've occurred to me to mention that fact in a query letter, because all that really means is I finished a manuscript and had the guts to send it out--a fact I'm proving anyway by querying! I did mention my membership in my local chapter and in the Beau Monde (the Regency special interest chapter--I write in that era, though with more battlefields than ballrooms), but only in the next-to-last paragraph after talking about my book and where I thought it would fit in the market. And I must've done something right, or at least not too badly wrong, because I have an agent now!