I see this is a difference between your style and mine. You're looking at fluff as just that, fluff. However in DFRPG a lot of the rules have to do with what is thematically appropriate. Does something perform a spray attack? If it's thematically appropriate it does (I.E. if the fluff is right). Does an aspect give you a bonus to this situation? If it's thematically appropriate. There are a lot of unfair advantages that being clever with the theme can get you, and it's exactly what DFRPG is about. Being clever with the flavor.
Yes, of course, but given people who describe their Breath Weapon one way the bonus to damage due to Strength Powers and not giving it to those who describe it a different way is still unfair and goes far beyond the use of aspects and such. It's very nearly given one group of people a free invoke of an aspect (or multiple free invokes) every time they use their Breath Weapon (certainly every time they hit). And you can argue a justification for a spray attack (which are generally rather weak anyhow) in either one of those two groups.
Aspects go beyond flavor and into rules, and certainly the flavor is part of the rules there. That's a good thing, and it gives power to the imagination, the creativity, of the players. Further, you can be creative with it at any time. A new person might have a lot to learn and it could be many sessions before they are using aspects well, but once they catch on, they can use it just as well as anything else. This is not at all the case if the GM makes a connection between the fluff of a breath weapon and whether or not it works with strength powers. The player gets tied to that description in a way that goes far beyond how aspects work, and they are stuck there unless they really change their description in what should be a fairly minor respect (and it doesn't affect anything else about what aspects they can use with the weapon, etc...generally speaking). That sort of thing should be avoided whenever possible, because it makes many players start worrying far too much about min-maxing descriptions than picking ones that they find cool and appropriate for their character.
I'd argue that breath weapons should either all give spray attacks or none give them as well. IMHO, as written none of them do (that should be something mentioned, like how claws get strength bonuses). But there's certainly no reason why any basic description of a breath weapon, whether it is something thrown, breathed, shot out of the eyes, or whatever, couldn't be fluffed into doing a spray attack if the GM says BW can do that (you only have to justify how they can hit multiple targets...easy enough).