The true test of a software sound library for arranger use is, can you substitute one sound with any other sound, and not have to do anything to it to have it sit right in the mix...

Most arrangers, you replace an acoustic piano with an electric one (a spur of the moment thing while you are playing, just for a change!) and there is no difference in volume. Replace one string sound with a synth pad, same volume... Etc., etc..

Replace one drum kit with another, same sounds on the same notes, roughly the same volume and dynamics.

This is something few sampled libraries bother with. You've got extensive work to do to balance things to the point where, if you call something up on the fly, it isn't going to jump out and bite you, or disappear into the mix.

Then you have the preset effects... same issue.

We've gone over the whole software sound banks/software or hardware arranger thing a bunch of times. So far, nobody has shown us anything that doesn't involve either serious compromise in what you can do on the spur of the moment (probably why you bought an arranger in the first place), or herculean work to set the software up to be as balanced and even as most arrangers are.

Most modern arrangers have thousands of sounds, dozens of drum kits, and they ALL sound great OOTB. You are going to have to do all that work yourself if you go with a software sound library...

Too much work for me, I'm afraid! Modern arrangers sound pretty damn good already - I'm not convinced the enormous amount of work to balance an entire software soundset is worth the gain in realism. If you just want a few sounds to expand the arranger's soundset, I guess it's OK, but if you want to replace the thousands that modern arrangers provide, better be prepared for a ton of work..!