in think there is a significant advantage to the pro edition because of, as PolarBear said, its ability to be reprogrammed - where i think an extra twenty percent can be had from this excellent library. even if the VIs achieve ten percent of that, there are still audible artifacts evident in the VI demos that reprogramming could fix if they didn't prevent access at the sample level. so certainly the VIs will raise the out-of-the-box bar, but even if that leaves just ten percent to be had, that's still the difference between A and B work.
In that sense I think the pro edition remains indispensible since it is the meat and potatoes of what we do musically, and will remain so in the SC. if time and money were no object, i'd try use a reedited pro edition along side the extended VIs (since there's no other choice), but the hardware and development costs seem so far out there to consider (the true cost of using VSL has never been the license). but i wouldn't think for a minute that the VIs wouldn't expand the efforts of some, just as it limits the efforts of others. it just depends where you want to be in all of that.
between innovations like synful, VSLs direct competitors and hopefully VSL itself, i think we're going to see some really significant things in the next 2-3 years, that makes just riding out the interim with the pro edition seem like a no brainer (and if you disagree with VSL's handling of the VIP program you may find upgrading at that time inequitable - especially if development of the VIs is abandoned or wrapped into yet another product like MIR). but under no circumstances would i think it would be bright not to have the pro edition if you could roll its cost into an upgrade if you really want the VIs that badly.