IMO, the unison technique is credible - and essential, especially when wanting unisons between I and II violins (as there is only one set of violin samples).
Certainly the full libraries allow for greater flexibility of expression than the SE libraries. But, the four SE libraries all told do contain what is absolutely essential. It is worth remembering that some of the highly regarded libraries that compete with VSL offer very few articulations (LASS for one). As for who buys SE, not everyone can afford to run a template of just the full libraries, or the computer power to run them.
As it seems that you are in the process of sorting things out, a couple questions to ask yourself might be: 1) What styles of music do you intend to write? Then, 2) What libraries will you need in order to write in those styles? Another thing to think about is: how much time to you intend to spend writing? Is writing something you want to do every spare minute that you have, or only for a couple hours a week? If only a couple hours a week, getting the full libraries might not be a real need.
Trying to answer #1 is important (to the extent that you can), as it will point you in specific directions. For example, if you were seeking to write primarily chamber music, you would have no real need to purchase AP Strings, but rather should get either solo strings, or chamber strings (or both if you could). Conversely, if you were most interested in writing large "Hollywood" sounding type scores, AP Strings would likely be very high on your purchase list, and chamber strings would probably be fairly low.
In addition, speaking from hard personal experience, the more you can figure out your goals in advance, the less likely you will end up spending money on libraries that in the end, you don't use (some of the first libraries that I purchased - from other companies - I now almost never use). One advantage of the SE Libraries is that they provide a broad overview of what is possible in the world of VSL. It is not uncommon for users to start with the SE Libraries, and then add AP Strings as their first full library purchase (not my own approach, as my main interests do not head in a "Hollywood" direction).
Also, if you go with the SE libraries, you do get additional discounts, when purchasing extended full libraries (and download libraries as well). If you have the budget, and know precisely what you are seeking to do, going directly to the full libraries can be a good choice. But, for the price of a couple full libraries (standard + extended) you can purchase all four of the SE Libraries, and have a much broader range of content. Think of the SE libraries as a type of introduction to the world of VSL.
Finally, what sort of computer power do you have or intend to get? Much less is needed to run the SE libraries than the full ones. The four SE libraries will run quite nicely on a Q9550 and 8 gigs of ram. To run a bunch of the full libraries, you would really need 24 gigs of ram (one can make do with 12, but that will be a real limiting factor), and at least a good i7 with 6 ram slots (the current Sandy Bridge processors have only 4 ram slots, tri-channel Sandy Bridge are due out before the end of the year)
To conclude, for my own situation, I can only buy a couple full libraries a year, and will never be able to afford to get VSL's entire catalog of full libraries (nor could I afford the needed computer power). While I hope, over time, to get full libraries for all the key sections, I will continue to need, and use, SE to fill in the gaps as needed.
Anyway, some things to think about.
