I don't think this will ever be straightforward because there will always be people who hear things others don't - placebo or not, who's to say?
I believe that much of the debate for better soundcards for digital links come from the early days where components weren't as cheap and easy to build as they are today. I guess back in the early days things were harder and more expensive to produce, less accurate, more jitter, distortion (ever heard your mouse movements over your speakers?) and depended on software to process certain things on relatively slow cpu's (whereas most things are hardware accellerated today). I believe these are things of the past, better components and cheaper production has solved much of these problems (contrary to analog outputs which are IMO still as bad as they ever were).
So to me, digital is digital and bits are bits. I've never been able to hear a difference between my Xonar digital outputs, the onboard, or any other digital output for that matter. As long as bits arrive, that is all there is to it as far as I'm concerned. From there on the DAC and analog components make all the difference.
This is pretty much my opinion on the matter. An external soundcard can be a huge improvement if you're using it as a DAC, but if all you're doing is sending a digital signal to a pre-existing DAC with modern equipment, I'm not sure there's much sound quality advantage to sending a digital signal to another box, to send a second digital signal to another box.
I agree with the posters above about the capability/technical issues: of course there are potential technical advantages to an outboard DAC if you have driver problems, if your interface won't let you send high sample rate audio any other way, or if your current SPDIF source isn't well-behaved, for sure. However, in terms of sound quality (as opposed to improved capability), I've never been able to distinguish two different digital outputs unless one of them was measurably broken. For example, I have three or four modern DACs around the house, and they all have USB, SPDIF coax, and SPDIF optical inputs. I've tried feeding those DACs with different source types (USB, Coax, Optical) and different sources of the same type (two different Coax sources, two different optical sources, etc.).
On any given DAC, I cannot distinguish digital input sources, at all. I can distinguish between the analog outputs of the various DACs, but that's because they have pretty radically different noise floors/output stages. Your mileage may vary, but I can't hear the difference between my onboard SPDIF output and the SPDIF output of a $750 external interface, when fed to the same DAC. The difference in the analog outputs of those two, on the other hand, is obvious to everyone I've ever played them for.
But, I think gtgray made a good point: if you can, try it for yourself and see. Borrow an external DAC, or see if you can demo one from a shop, and try using different digital inputs. Make sure you're comparing apples to apples, but there's no substitute for experience.
I'm also of the "digital is digital" opinion but couldn't get my head around why external soundcards are globally touted as being superior to onboard sound, if they have digital output and you're feeding another DAC with it. It's just an unnecessary link in the chain. It's obviously just the DAC component that's being referred to as being superior and the choice of sound card depends on whether it will do a better job that the AV Amp that is at the end of the chain.
I agree. I think the reason that external soundcards have a good rep is, in part, because legacy digital audio was kind of dicey (as Inflatable Mouse noted), and, in part, because you can buy externals with better (and better isolated) DACs than most AV Amps have. I think, with modern equipment, sending a USB or SPDIF signal to an external box so that it can in turn send a SPDIF signal to another box is (in most cases) very unlikely to improve sound quality, and could, potentially, introduce additional jitter (depending on how the pass-through works). Obviously if you have specific technical limitations you're trying to overcome, that's a different story.