The first, and primary reason any beer comes in branded glassware is this: Marketing. Brewers, like any other businesses, like for the general public to know that the anonymous tapped beer you're drinking is theirs. That is why they provide branded glasses to bars, and request (or require) that their beer be served in their glasses.
Is there a benefit to the mildly curved shape of this glass for this type of beer?
In my observations, yes, there is. While the throat may not narrow as severely as a brandy snifter, the effect of concentrating the aroma is still achieved. Eyeballing it, I'd say that the curvature is somewhere in between that of my champagne flutes and my red wines glasses, which are presumed to have the same general effect. More generally I'd say that a glass with a narrowing or restricted throat does concentrate the aroma, and the degree of restriction doesn't seem to matter significantly, to my nose. (Assuming it isn't too great, that is. Beer in a bottle or can, for instance, is definitely at a disadvantage.)
I'd like to also look at a few of the reasons they state in their marketing material as well.
The authentic shape of the body encourages the perfect balance of C02 and liquid, enhancing head retention and flavor.
I don't have any evidence to back this up, but I suspect that this is probably the most legitimate of the claims. This would have a significant effect on the experience of drinking the beer, it is going to be specific to their recipe, and it's a reason that's often given for custom beer glassware. (Boston Beer Company laser etches the bottom of their Sam Adams Boston Lager glass to enhance cavitation, for instance.)
The angles at the bottom cause a wave that works in harmony with the liquid alchemy.
I have no idea what this means. Alchemy is the term for pre-chemistry, and primarily deals with trying to turn ordinary substances into gold. I assume they're trying to insinuate that the glass magically turns Stella into better beer.
The stem provides a means to hold the chalice so that the Stella Artois stays cold longer
This is plausible, and perhaps unfortunate. This is why you are supposed to hold red wine glasses by the bowl (to warm the wine) and white wine glasses by the stem (to keep it cool.) However, with wine, a serving is generally five ounces or so, compared to twelve or more of lager. The effect certainly won't be the same given the significantly higher mass. The unfortunate bit is that colder is not generally what you want when you're talking about good beer. Colder is the realm of the cheap macro-brews like Coors Light. Cold subdues flavor compounds, which is useful when the flavor of the beer is nasty, and not so useful when it's pleasant. This certainly isn't a a requirement or general best practice for lagers as you won't find stemmed glassware used for Boston Beer's Samuel Adams Lager glass:
or my classic Spiegelau Lager glass:
or any of the glasses you'll see used for the many fine Lagers of Germany. So, I have to suppose one of the following two reasons. First, they may simply be pandering to the general belief that colder beer is better. Second, it would be unfortunate if they were trying to hide a lack of quality by trying to ensure the beer was too cold to taste properly.
Finally, and I've left this to last, because it's purely my opinion...I think they've chosen the chalice glass specifically because it's fancy, and that's the image they want to portray for their beer. When people see the Stella glass, they don't want their beer to be lumped in with the pint-glass styles, they want it to stand out as sophisticated, and the chalice specifically gives it that cachet.
So, my top line and bottom line are down to marketing, with a bit of truth and a bit of fiction to all the bits in between.