A couple (well, four) possible reasons for starting with SE rather than going straight for the Cube. YMMV obviously:
1. Cost: I couldn't afford the Cube when I bought VSL all those years ago. I bought SE, and when I had the extra cash upgraded as and when. Probably more expensive in the long run, but ...
2. Needs:The first instruments I upgraded were flutes, oboes and horns, because they tend to be the featured instruments in the kind of music I write, so I wanted the extra articulations / velocity layers. However, I still to this day use the SE versions of the bassoons, contras, tubas and some others because in the music I write those instruments tend to function in a supporting way. I only need legato / sus and staccato articulations, and the extra velocity layers, etc. wouldn't be noticeable. So again it's cost, but also the SE instruments take up less memory and keep the size of my template under control. If I decide to write a tuba concerto I can always upgrade that instrument later.
3. Pick and Mix. I love VSL, but when deciding what libraries I wanted to use I preferred LASS for strings, EW Symphonic Choirs for voices plus some other non-VSL libraries I already owned and liked. Buying the Cube would have meant paying for VSL instruments I would never use.
4. Test Driving. After working with SE for a few months I found I was struggling to get the brass sounds I wanted for a particular project. VSL is very powerful and flexible and probably, if I'd upgraded to the full versions of the instruments and put the work in it could have delivered the sound I was after. However, EW Hollywood Brass gave me that sound out of the box (and was on sale so was cheaper than VSL). SE let me audition VSL instruments and decide what IMO their strengths and weaknesses are, and how they compare with other libaries. (And I have to say, regarding brass, if I was writing a more classically inspired / symphonic piece I'd probably go back to VSL. It's just for my current project I'm after the Hollywood sound which I found easier to get from Hollywood Brass. But SE let me learn when to use VSL, and when not.)
So, for me, if you can afford the Cube, and afford possibly to be paying for stuff you might not ever need (hundreds of bass clarinet articulations when you don't normally score for bass clarinet, preferring bassoons) or because you, for whatever reason use other libraries as well, go for it. If not, consider starting with SE and building your template oragnically over time.