I mount scopes for a living and a one piece picatinny can have one huge advantage. With so many different tube lengths in scopes now there often is not enough adjustment for proper eye relief with standard two piece bases. You then have to use an extended front base or extended rings depending on the platform. I see this most often in the Remington 700/Howa 1500/Remington 700(they all use the same bases) and Savage. With a picatinny you have so many more options of where to place the rings as was previously mentioned. Another big advantage to a one piece base - picatinny or Weaver style - is if you have to shim it doesn't stress the scope like shimming with two piece bases does.

I disagree with with always lapping rings. Warne expressly advises not to lap their rings and I've never had an issue with them. And Warne mounts and rings are what is on seven of the nine non AR rifles I own. The other two have Leupold.

Not claiming to be an expert but do mount 15 to 20 scopes a week.