Rendering differences is the reasons. To get the site to look consistent across all major browsers and platforms Cufon is the only practical option at the moment.
The licensing issues is a problem of its own even with image replacement... even if it's a server-side image replacement like Flir. Many foundries are still scared sh#tless of the Internet and are overly protective of their work. Some are not, for example - surprise - Adobe and Dalton Maag. For other fondries however it's a business decision. For example, one cannot use any of HF&J fonts such as Gotham and Archer online in any form or fashion. They are building their own font service. Anytime you see a site sporting Gotham under Cufon, it's an explicit license violation.
So in short - if you are considering using a font with an image replacement, check the license. If it is not explicitly permitted, then in all likelihood it is disallowed. If still set on using this font, email foundry and ask.
You mean I can't automatically embed HFJ faces, right? If I painstakingly Photoshop my hedlines, I can use Archer for that, right?
Also, when implying that font-face has dealbreaking rendering quality issues, are you referring to their performance for setting body copy, or do you think it flat out doesn't work for display type either?
It's not so much that rendering a whole bunch of text takes a while (though it might, but it probably has more to do with the time it takes to download the font), but more that the letters look different depending on who looks at it.
After that, it's up to the designer to decide whether or not the difference is acceptable. I'd say it works out fine in pretty much all cases. Your site doesn't have to look the same in all browsers, people.
> Also, when implying that font-face has dealbreaking rendering quality issues, are you referring to their performance for setting body copy, or do you think it flat out doesn't work for display type either?
As of today, there seem to be very few fonts, even from the specialist services like Typekit, that render well across all browsers via @font-face. In my experience, this does apply to both body text and headline sizes, but what you might call the risk/reward ratio is different.
For typical body fonts, there is never going to be a huge difference in appearance between text set using @font-face and text set in well-established screen-friendly fonts like Verdana, Georgia and the Microsoft C-fonts, at least not until we get the kinds of resolutions now used on smartphones routinely available on full-size screens as well. IMHO, using @font-face for body text seems like asking for trouble today: poor hinting or antialiasing can literally render your text illegible, and there is hardly any potential upside anyway.
On the other hand, for headlines, pull quotes, and other text typically set a bit larger than body copy, you can get some genuine character in the font on a typical display today. There certainly are still problems -- many trendy graphic design blogs currently look terrible on my screen because they are trying to be too clever with this technology -- but the flaws are less significant relative to the overall effect, because things like antialiasing and hinting don't matter so much at larger sizes, and it's very unlikely that your text will actually be unreadable as a result. That means there is more incentive to use different fonts via whatever mechanism when you're setting larger text.
> You mean I can't automatically embed HFJ faces, right? If I painstakingly Photoshop my hedlines, I can use Archer for that, right?
Yes on both counts.
> Also, when implying that font-face has dealbreaking rendering quality issues, are you referring to their performance for setting body copy, or do you think it flat out doesn't work for display type either?
Depends on the typeface, or more specifically on whether it was explicitly tested and "optimized" for @font-face use, which typically refers to generating hinting info needed to make the font render decently on non-Apple platforms. For example, Museo will render great in all browsers, while something like Avenir will look bloody awful in smaller sizes in FF (not that it is available for online use to begin with). This narrows down the list of usable @font-face typefaces quite a bit, it also complicates testing and what not.
The licensing issues is a problem of its own even with image replacement... even if it's a server-side image replacement like Flir. Many foundries are still scared sh#tless of the Internet and are overly protective of their work. Some are not, for example - surprise - Adobe and Dalton Maag. For other fondries however it's a business decision. For example, one cannot use any of HF&J fonts such as Gotham and Archer online in any form or fashion. They are building their own font service. Anytime you see a site sporting Gotham under Cufon, it's an explicit license violation.
So in short - if you are considering using a font with an image replacement, check the license. If it is not explicitly permitted, then in all likelihood it is disallowed. If still set on using this font, email foundry and ask.