Actually, not much!
True, your first or second "demonstrator" is trilling to fill and interesting to look at, but there can be a downside when you consider their value. Hate to be a wet blanket here, but when you consider what makes an expensive pen, expensive, one of the items on that list is the so-called "precious resins" they're typically made of. Jjust how precious that resin is in reality, is generally up to the manufacturer--and few of them will say it's cheap, and so, by implication of their MSRPs, we have no choice but to take them at their word.
On the other hand, when it comes to clear pens, the choices of material are fewer and always less exotic--after all, clear plastic, is clear plastic (lucite is lucite, there's just no getting around it.) But if that's the case, why are we paying the same price (or possibly more) for the so-called "demonstrator" than we are for the pen made from precious resin?
I'll tell you why--it's because we are being hood-winked! I say a Pelikan M800 made of clear plastic is much cheaper to make than the snappy striated red, green or blue ones we typically see. Or how about a Visconti Medici Rose Gold, "utilizing the same innovative Acrosilk resin" combined with 18kt rose gold trims. That pen retails for just south of $1,000--should we pay the same for a clear plastic version? The guts of either are the same, it's only the "wrapper" that changes, right?
So buy yourself a nice demonstrator someday and enjoy it's fascinating innards all to hell--but don't make a habit of them--they aren't worth it the prices we pay for them.
Fortunately, there are exceptions--buy a TWISB! As a mater of fact, buy a lot of TWISBs--they've always been clear and they've always been reasonably priced--and their filling systems are just as interesting to see and operate as 99% of the more exotic pens and expensive pens out there--be they clear or otherwise!