I guess, for me, the whole point of an arranger, versus a full-blown workstation, is the instant gratification aspect of it. I have a fully loaded Kurzweil 2500S and a huge library of sample disks, and indeed you can do incredible things with it.
But....... if I want to grab a keyboard out of a box and go gig with anybody, anywhere with no preparation time, I'll get an arranger from one of the top three (and everyone knows my liking for the Roland OS and form factor!) every time.
I can do everything that Mediastation does (mostly) with my home rig with computers and software, along with the K2500 and G70 for extra sounds, but on a gig, to be honest, both the T2 and the G70 have all the capabilities that most live musicians need, and with a minimum of setup and learning curve.
As I said, I believe the computer/keyboard hybrid is the way of the future, but one of the current roadblocks is a sound-set as well developed and tweaked as the best of Roland or Yamaha. Last video I saw of Mediastation in arranger mode, it was triggering the Hyper Canvas VST, a toy compared to the current top of the line Roland sound-set, yet alone the T2's incredible voice selection.
Developing a coherent, comprehensive new sound-set seems to be almost beyond the capabilities of Yamaha and Roland (now that Roland have semi-abandoned the Sound Canvas concept for their G and E lines) and no software plug-in that I've heard yet offers as many different sounds, and more importantly, as well balanced a sound-set.
Remember, it is not sufficient to have a great alto sax sound and a great tenor sax sound (for example)...... they also have to be interchangeable, with an equal power and equal eq requirements, equal 'touch', all the things that make it so, in the middle of a solo, you change from tenor to alto and don't have to make any other adjustment.
Now add in the drum-kits, the different basses, ALL the myriad sounds we all need, and the job of balancing and making consistent is herculean. Even Yamaha (IMHO) never got it as right as the Sound Canvases did. You play an SMF from the original Sound Canvas on any Roland arranger up to the G1000 (the last to use only Sound Canvas samples), and no matter which newer sounds from the G1000 you used to replace the original ones, the balance and eq were still spot on.
This is the achilles heel of the computer/arranger... finding (or developing) a coherent sound-set that rivals the built-in ones of the top three, if the available commercial sound sets have difficulties combining wide choice with wide compatibilities. The big three have had decades to develop their sound-sets, and as of yet (once again IMHO) I haven't heard a commercial sound set that rivals them for across the board compatibility....... (and even Roland, now they have abandoned the Sound Canvas concept, are harder to voice as interchangeably as they used to).
It's easy to add a great Giga piano library, or Giga bass library, etc. to a Mediastation, but it's a far more difficult task to set them up so that they can be called up at any time to replace another sound, and the volume will be perfect, and the eq will be perfect, etc., etc..
Not so much a problem at home or in the studio, but on a gig there is no excuse, no time to tweak, no time to go home and balance it..........
An arranger is just a tool. What matters is what you build with it..!