Probably you are referring to the VC-1 expansion card, a card for the Roland V-Synth and the Roland VariOS which is essentially a D-50 on a card. It's more or less like the PLG150-DX card for yamaha synths (a DX7 on a card). This makes sense because both the DX7 and the D50 (but especially the DX7) are two of the most successful synths of history, and it might be better to have them "fitted" than to try to buy the real thing, because they're old machines, they are big, they are quite expensive, they will likely be prone to breakdowns due to its age and old technology, etc. On the other hand, those expansion cards sound in fact better than the original, because they take advantage of the better DACs of the host machine (it's known that the DX7 didn't have very good DACs and the sound was quite noisy and grainy; this was fixed in later FM synths).
As I said, I think that those expansion cards makes sense when talking about real classics such as the DX7 and the D50 (BTW, yamaha has also the PLG150-AN, some kind of re-creation of the Prophet 5, one of the best and well-known analog synths), but it would be too much work and investment to make cards for many other and less successful synths; it's better simply to look for the real thing.
In my experience, the VC-1 card for the V-Synth does not live up to the real D-50's sound. The reason for this is the DACs. The D-50's are somewhat flawed by being just a bit noisy, but most feel (including myself) that this lends to it's unique sound and character. The same holds true for the DX-7. I just recently bought a Sequential Circuts Prophet 600 synth (the first consumer synthesizer to have MIDI), and the sounds one can get from that are amazing. I then tried a emulated version on a card pluged into a Yamaha S80, and it sounded terrible.
So, I agree with shad0fax, buying the real thing is the only way out. One analog synth I would love to have would be the Roland Jupiter 8
, now theres a awesome analog right up there with the Prophet 5.