Hi,
I turned it off 3 years ago. Since eBay has stated they intend to turn-off 3rd party checkout, it probably makes sense to move to eBay checkout anyway. They made that statement last year, and the only slow-down is the 3rd party systems did things they couldn't. Now that Propay is onboard, they'll probably give a date for cutover soon.
The big draw for using Auctiva checkout is/was the Merchant Account capability.
If you turn it off, it doesn't affect active listings, only new listings. That might cause a problem with any combined situations for checkouts with listings in both systems.
Danno