You can tell you don't really know this synth that well, for sure. Also each layer DOES have its own volume (Program volume) in the menu, one press and a scroll away. I think you, and some others, really need to focus on what REV2 is rather than what it isn't. Then you'll see the beauty in like I and others do. I have no issue with the ARP (other than lack of beat sync) and the sequencer is fun, instant and does the job I need from it (quick riffs and song ideas for later). I didn't buy rev2 to get a fully featured sequencer, these are all great bonus items on a very powerful synth with almost every toy you could wish for in a 5 octave modern full analog synth FOR THIS PRICE!
The combine osc mix knob is better for morphing/fading between with ONE MOD (or finger move) vs trying to adjust one down while the other goes up + all the trickery involved in doing that smoothly with two knobs. I think you aren't getting that it's not prophet 6, it's far more complex and powerful in terms of engine and thus it has to do things certain ways to allow it to be very usable. Prophet 6 has what it has because it's a VERY simple synth by comparison.
I've done loads of fading between layer patches (mod wheel or whatever) and you can also use env amount on filter or cut off - modulated - to do similar rather than just pure volume). I really don't get all this nit-picking, the rev2 has so much going for it and so few drawbacks vs almost any other synth in its price range (full analog) old or new, that includes prophet 6 and OB-6 which cost even more and have dwarf keybeds! I've played with prophet 6 many times and it never blew me away as a synth, only as a cool sound source but not as an overall empowering instrument that can last a lifetime and do so much. I don't even love it's sound over rev2 but to find that you need to get to know rev2 and program to its strengths sonically, while Prophet 6 is more instant of course but also more limited and often 'samey'.