I experimented quite a bit last year with a chopped-down stubby freestyle fin. I took a pretty big old MFC Freestyle fin and chopped it down to 18cm, then ground back the material so it wasn't quite so thick and stiff. I sailed this quite a bit with my 105L Goya X1 and sail size from 4.7 to 5.3, mostly over a two week stay in Cape Hatteras in spring 2008. Here is my verdict.
First off, some background: I am old and stiff. Being from Kingston, time on the water is limited and this only contributes to being stuck in the slow-lane on freestyle progression. I've been sticking vulcans for years and spocks & grubbies for several, but quite inconsistently. I am working on tricks like flakas and shakas, and continue to put in the hours on the spocks/540's and grubbies. So, while I am working hard on spinny flat-water freestyle tricks, I am not a pro and not going for extra rotations... never mind switch stance, clew-first or one-handed.
My diagnosis is this: First and foremost, getting and staying upwind becomes a challenge. Why is this a problem? Well, maybe it's just me, but as a freestyler, I find there is considerable benefit to being on the smallest sail possible, yet sufficient to get me planing. So - ultimately, this translates to being under- to moderately powered, but rarely overpowered. Hence, already the lower power available dictates that upwind sailing will not be straightforward. Throw on a stubby fin and there is significantly compromised ability to angle upwind during reaches. What I have noticed however is that because of these apparent limitations, it vastly changes ones stance and approach to getting upwind. The standard approach tends to be pressuring the fin to deliver and maintain that upwind angle. However, with the stubby fin, you tend to adapt your style to rely much more on the rail of the board to cut in and grab the water to help you keep your board angled up wind. One might ask: Why do you need to go upwind? The answer for that is easy. Watch any pro video of almost any modern freestyle move and you will immediately notice that they are executed on a downwind reach, anywhere from subtle (spocks) to extreme (ponches). So, if you are doing relatively short reaches to maximize your transitions (when and where you get to try your tricks), then you spend an awful lot of time heading downwind in the transitions and aggressively upwind in the reaches. I suppose that is why freestylers might tend to upset the standard back-and-forthers (BAFs) out there - we are all over the frickin' place!
Secondly, did the fin help me with my moves? Short answer is no, it did not. While a massive slalom or freeride fin (25-30cm) in that same board would create significant spin-control issues, I typically run a 23cm MFC freestyle fin anyway. In the spectrum of fins, freestyle fins are generally on the small side already. So, the typical 23cm fin I use doesn't really discourage board spin like say a 28cm swept slalom fin does. Note that I have actually played on the odd board with a slalom fin since my spock days and you really, REALLY have to watch out for how fast those fins re-catch and want to spin the board once you are sliding backwards. This is actually quite dangerous, since if you don't spin your intial airborn part of the spock around enough, it is almost as likely that the fin will catch and the board will want to spin back the way you came!! This can result in 'rotini-body' and excrutiating back, leg and ankle twist. Even when you do spin enough and the rotation continues in the intended direction, the board spins very VERY fast. This is definitely a benefit of the relatively small and straight freestyle fins - a sailor has considerably more control over the direction and speed of the boards rotation, or at least, so it seems for me. 
So, what is my opinion? Well, with regards to those stubbies - personally, I think they have considerable benefit for sailors at the pro-level, in particular, for those less challenged by the compromise in upwind performance and those needing to 'cut-out' or 'spin-out' the fin to accomplish a second rotation. Certainly, when you look back at the archives on "The Future" and "double-flakas", they are not actually doubles, to be precise - they are often a single spock/flaka with a second on-the-water rotation (not meaning any disrespect - incredible moves far beyond me!). In this situation, obviously a stubby fin is beneficial to make that fin release easy for the second rotation. Nowadays, when you see a triple flaka, like this one by Golitto, he actually gets airborn like a true flaka on two of those three rotations. Incredible really. Now it would seem, the benefit of the shortest fin possible is that it is easier to get it out of the water (as opposed to spinning it out). Summary: Using a stubby follows a trend that pro's have demonstrated works for them. For the average aspiring freestyler, I don't believe the benefit is worth the trade-off (lack of upwind-ability: less opportunity to try tricks!). Saying that - it is certainly fun to experiment, recycle those old fins and figure out what works for you! Now I wonder what a purpose-built stubby would feel like, perhaps like this little gem in an 18...
