Well, it's all relative really, we've now been conditioned by the 1199 to view the flat-rate linkage as a requirement for the track, however how many other bikes now come stock with a flat-rate linkage? None that I can think of. In fact, the only prior Ducati Superbike that came with one was the 749R. We had to add an aftermarket flat rate linkage to the race/track bikes on the 749/999 and 848/1198 models.
The 1199 linkage will bolt right on the 899 to get a flat rate for the track. I doubt many riders can tell the difference between progressive and flat on the track, but the difference in tire wear should be readily apparent for anyone who's riding hard. In general, that's something that I'd recommend for an experienced/advanced track rider or racer (who's class allows it) or someone who aspires to be at that level and wants their bike set up optimally for the track right from the start.
I haven't ridden my 899 yet, still middle of Winter here so won't be back at the track until April. I'll do a full write up comparing the 1199 and 899 as soon as I have some lap times in decent conditions to compare.
P.S. - If anyone needs a flat-rate 1199 linkage to put on their 899, shoot me a PM, we have a whole pile of them that we've accumulated.