I know the argument for live fonts in banners. Easier to make client changes. Less file size. Crisp readabilty.
But unless specifically told to do so, I will never use live fonts for the following reasons:
1) Images are much faster to code. Live fonts require a while bunch of CSS to get it looking correct. Imagine a price point, with a small $, different size dollars and cents, and a few dividing lines for good measure. Not difficult CSS, but takes way longer that just saving an image in Photoshop. Just set up an action in Photoshop to turn a layer or selection into a transparent PNG, and thats your image. And as long as you keep the PSD, it's easy to make changes. The one thing I'll give live fonts in this area is its easier for a new developer to take over as you don't need to worry about passing along the PSD as well.
2) Images are much easier to QA. An image is an image in every modern browser and platform. No way can you say the same thing about live fonts. It will looks slightly different all over the place.
3) Unless you have a web font licensed, you will need to resort to Google fonts, or something else not quite what the AD wanted. And even if you do have a proper font to upload with the zip, it will never look exactly the same as the PSD. Sometimes very close, but never exact. And the AD worked hard at all those kerning and letting settings which will be tossed.
4) Custom fonts can be huge, thus offsetting the file size you saved from using fonts instead of images. Maybe even putting you over the file size limit. And you will usually have to load it from the cloud, which is still a grey area for many ad servers.
5) Using tinypng.com with retina sized PNGs is surprisingly efficient for file size and quality. Thus quality with images will not be compromised.
Overall, the thing about banners is that they are temporary venues for advertising. They will typically be handled by one dev only, and only be around for a limited time. There is no need for easy editing or ease of transfer. File size, image quality, and speed of development are the three key aspects. File size/quality is debatable depending on the design and fonts, but usually can be done with images no problem considering what banners are usually designed like. But speed of development, there is no argument at all. Images are way easier. And will always get you exactly what the AD (and client) is expecting without having to resort to "Well, it's HTML text so it will look a bit different than the PDF."