Significant? I think the advantage for fuelly in having an app in the android marketplace is significantly more exposure.
You have several types of exposure,
#1 people that will roam the marketplace for 'fuel' 'car' 'efficiency' 'mpg' 'history' (you get the picture) applications, you'd be surprised.
#2 People that will see the icon on your homescreen and ask about the app. Ok well yours or mine or anyone's I guess. In the interim my url link has your icon so you still get this benefit.
#3 People that app share or blog regarding Android apps. I share a list of mine weekly with coworkers just to see what's new and interesting.
I was looking for an app that did exactly this when someone mentioned the site and I joined. I'll mention it to others and shoot a link, but an app is just a nice way of having presence.
Still listening?
The m.fuelly page is clean and fast, and seems to load quick on my verizon Droid, nice job.
As has been stated...with an app, the app could database the info and send it up non interactively in the background once the user has hit 'ok' and locks their phone again making it just a little bit faster.
Who cares? well this means I type it in while standing there at the pump in 2 seconds before getting in my car, as opposed to getting in my car, remembering to load the page, enter the data, hope it sends/hope I don't forget, etc. User habits are a huge part of why something like this (I think) would be to your advantage.
I hear what you're saying regarding apps, app stores, developers, and api lifecycles. I can't write to save my life. But if you're looking for more users, think of an Android app as free marketing more than just a convenience / geekdom item.
Funk 
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