I bought my 510 at a Home Show in 1991 after lusting for a Shopsmith for about 5 years. The versatility of the machine and proficiency of the demonstrators either makes you love them or hate them, in my case it was the former. Unlike what I read about many users, I use the tablesaw aspect most and since most cuts are not with a tilted arbor (or table), I don't find that the tilting table issue to be much of an annoyance. What I did find to be troublesome was the 510 fence was difficult to align properly and it also would kick out on the outfeed side. When I saw the 520 fence at mall demonstration, I liked the improvements and upgraded to it. This was before they SS added the scale feature to the front rail. The 520 fence worked well, but I still found it to be a little short and I really wanted a rip scale to avoid having to measure all my rips. Adding a fence extension would easily of fixed the short issue, but there is also the issue of you may or may not have table under the place where you want the fence. I always (well almost) connect the main table to end table with extension tubes to limit the twisting of the table and there is usually about a 12 inch gap between the two tables.
So when I saw the the jointech fence system at a woodworking show maybe about a year after the 520 upgrade I had a mental debate about the impact it would have on changeovers and ended up buying it. First off the Jointech is a really nice fence system for the router, but you can have that without putting it on the SS. I liked the added table surface and the ease of adding a very accurate rip scale along with a longer fence. In use on the SS I found the added table too much of an annoyance to remove all the time durring changeovers. Also because of the new rails it really left you without a fence for the drill press and the occasional tilted table rip. Jointech says you you can use the feather board carriage for these operations, but I did not find that to true at all. The feather board carriage has no provision for alignment other than how you place it before tightening it to the rails, so for the drill press it worked ok, but was totally useless for beveled rip cuts. So I put the 520 rails back on the SS and have been more or less happy, reserving the Jointech for my router table.
It seems like I always buy too quick as I like the setup Jointech has now where you leave the 520 rails on the SS. I think the router table is better served off the SS but it would be nice to able to mount either the 520 fence or the Jointech fence to the SS for tablesaw operations. So now I am finally to the question, Why? I already mentioned the longer fence, but the rip scale is excellent, it takes seconds to zero as you extend the fence out to zero and slide it up to the blade and lock it in, now you have a very easy to read and set rip adjustment and with the micro adjustment you have more control than you need but you still find yourself using it. I envision using 5' tubes mounting the Jointech on the small table and putting a floating table between the end table and main table to adjust for rips.