Yeast cant eat (metabolise) sucrose, it cant transport it across the cell membrane like it can Maltose. So it excretes invertase into the wort, the amount is in direct proportion to the amount of sucrose (the more sucrose the more invertase). The invertase splits the Sucrose into Fructose and Glucose.
Yeast also metabolises sugars in a specific order Glucose and Fructose first, Maltose and higher saccharides pretty much in order of ascending molecular weight.
A couple of things happen in the process in response to the wort composition, there is always some simple sugars in a well made all grain wort; Hexoses (mostly Glucose and Fructose totaling about 7-9%), quite a lot of disaccharides Maltose 43-45%, Sucrose 3-4% Maltotriose 11-13%
Yeast needs energy up front to start doing its job, so the readily available sugars are used first. strangely yeast will give more metabolic energy to excreting Invertase than to importing Maltose, so a relatively small addition of Sucrose can cause a surprisingly large change in the yeasts behaviour, and as Manticle said it is thought higher Invertase levels that stay in the beer until we drink it is believed will affect the flavour.
Too much Sucrose can actually inhibit the yeasts ability to import and metabolise Maltose, propagate yeast on a high sucrose diet for a couple of generations and yeast can forget what to do with maltose, which will really bugger up your beer.
There are also a number of other effects, not all just related to the Sucrose : Maltose ratio, when sugar makes up a significant fraction of your extract, it also affects the amount of protein, FAN, higher saccharides, vitamins and minerals... in the wort, all of which change what the yeast makes from the wort.
I suspect the amount of things in raw sugar that aren't Sucrose would be having little effect on the flavour (there isn't much). If I wanted to make a "commercial" lager I think I would keep any sucrose under 10% not add it until part way through the ferment, have a look at your mash regime design, aiming toward a higher attenuating clean finishing beer.
Personally I think you can make a really good summer quaffer without adjunct and that its a beer I would rather drink.
Mark