I like double auto-ratchets for fun sailing, and double ratchets are a must for distance racing. However, the double ratchets can cause problems when dousing in buoy races: If you step on a spin sheet, you won't feel it with the 100:1 advantage of double blocks.

Folks with separate tack lines for their spinnaker should mount the forward block inboard of the aft block to prevent the tack line from getting sucked into it. Doing so also shortens lines and keeps lines neater especially on side-laced tramps. At least that's my theory after seeing that Lovell and Ogletree do it that way and we have regular trouble with the forward blocks immediately ahead of the aft.