Hi Rebecca! In my opinion ribs on a snaffle isn't a bad thing, it could be help to encourage the horse salivation and emits a pleasant taste and also helps to encourage lightness for horses that lean on the bit!
I definitely would not advise any bit with raised ribs due to the ribs adding additional unnecessary pressure (causing pain) to the horses tongue and gums. This in turn would result in a more tense and resistant horse. Stick with what you have if he goes well in it or go for something more lightweight.
I wouldn't be concerned about him having a dry mouth when riding, though any metal on dry gums is likely to cause more discomfort to the horse. Horses salivate when they are relaxed or are preparing to eat - not when they have metal in their mouth. You'll see some horses salivating with a bit due to their brain confusing the bit with food, which is only instinctual.
Well, you have my opinion on the raised ribs on a snaffle and hopefully some information on why your horses mouth appears to be dry when riding. Hope this helps! :)