Just curious, in the BBQ setting, why would you pick a griddle over the grills? Or is it just for specific recipes?
In a charcoal bbq, you'd probably loose out by using a griddle, less smoke and radiant heat exposure from the coals. Basically like putting a CI pan on your kettle bbq.
On a propane or NG BBQ, you don't get any wood-smoke flavour, and only
some flavor from 'drippings' that have flare ups causing a
kind of smok-ish flavor, but I've never found it very prevalent. It's part of why purists crap on gas bbqs compared to charcoal. An advantage though is that the grills are basically the easiest thing to clean. Heat and scrape and you're done.
A griddle helps you get a better sear in the same way cast iron does, and also lets any meats soak/cook in their own juices instead of them dripping bewteen the grates and down into the bbq, at the expense of no exposure to flare ups so a guarantee of no potential extra 'grilling' flavor. The downside is cleaning is more difficult and it's usually a lot more money for a griddle compared to bars as it's a lot more steel.
I think there's an argument for both. I never felt the 'flare ups' add any real flavor compared to a wood or charcoal cook. I'll likely still use both for different things at different times. You're right that some recipes it's basically a must for a griddle though. For smash burgers, a really good even sear on a steak, or teppanyaki style cooking a griddle is better (or the only option). Same thing for eggs, hashbrowns, etc.
For some basic burgers, hot dogs or skewers, I'd probably stick with the grills, or if I want the visually nice 'X' pattern on a cooked steak. Fortunately on my Napoleon the infrared side burner is still there so I always have 'grill' too. It's easy enough to remove the griddle and put the bars back in that it's not an issue. The stock Napoleon pro bars are very thick stainless so they do a much better job than the cheap thin steel ones on some bbqs.