As an owner of a 70's Chamberlin, M-400, an M-400MkVII, the M-4000D and the all the software, here's my spin.
First it's very difficult to get any software to operate reliably "Live" on a stage. And if it shuts down or your computer locks during a performance its a sad event. (Been there; too many times) That alone justifies the price for some (like me).
Also, there is no comparision to the sound bewtween the software and the M-4000D. The Mellotron Archives, the souce of the M-4000 has ALL of the orginal MASTER tapes from Mellotron. The first generation tapes from the original sessions. The software guys depended on actual Mellotrons to make samples of, and if you know Mellotron's that can be problematic depending on their age and condition of both the tape transports and the tapes of the machine being sampled. Also they are samples, and the quality of sampling depends very heavily on the quality of the engineering.
For me the sound quality is essential. Firstly, I'm an engineer. But I play too. My keyboard rig uses hi fidelity playback gear, SSL mixing (X-Rack, Panda), Crown, EAW, etc. It's a Pink Floyd tribute band running a "quad" system using an Avid/Digidesign Digital live console. With this level of sound, you can readily hear the difference. (BTW we do play some old Genesis once in a while ergo the Tron). I do run software (on a reliable Linux machine), but mostly it's for sequencing (I only have two hands and two feet) and automated mixing, and midi program changes, as well as CV control for the analog stuff I have (Moog/Dotcom Hybrid Modular, EMS Synthi A, etc.). Not for sound sources. None of my keyboards, except the M-4000D are digital, virtual or modeling types. All analog, but with digital control. It makes a difference.
Remember ... your ability to hear a difference is very dependent on the gear you are listening to it on. A Roland keyboard amp just isn't gonna give anywhere near the resolution one needs to hear a difference. Same goes for a Mackie mixer and a pair NS-10's, or most of crap being sold cheap to musicians these days. This junk just smears everything into one giant bad sounding mess anyway. It all (no matter how good the sources are) ends up sounding the same ... bad. And don't kid yourself, not everyone listens to music on an Ipod or phone with MP3. Many people actually have good home audio, so don't discount your fans.
So anyway ...
In some repects, the Mellotron M-4000D sounds better than my 70's vintage Mellotron M400 which is in pristine condition, and has all the upgrades. The old Mellotron tape heads create like a mask over the sound if you know what I mean. And hell ... It weighs next to nothing compared to the M400. I had another nice M400 which I used to travel with in a huge roadcase, but first I went to software for onstage (big mistake), and now I have the M-4000D.
Oh, and by the way, the oddities and the hiss are part of what makes a Tron a Tron. An M400 doesn't always play a note same way each time you hit a key. The tape wobbles a bit, or doesn't align properly, or doesn't return to the beginning fully, which gives it part of its character. And this is something a real Tron player uses. Kinda like using the side of a pick on a guitar to get a harmonic squeal. It's an instrument, which should have some human input. Which is the one thing the M4000D doesn't do well. It's a bit to perfect in that sense. However, it "feels" remarkably like a real Tron.
But here's the bottom line. You are paying for an instrument. An instrument has human influence. Now ... if your a Pohducer (yes that's the spelling I meant to use) running midi tracks it doesn't matter what junk sound source you use. You're not using instruments, you're using sounds. You are not "playing" anything. But those of us who play, have a tactile influence on our music, and also play live a stand-alone board that does what the M4000D does is worth the price. Most of the "you can get same sounds in software so why spend money" crowd probably don't actually nor have ever bought or played a real instrument, like say a piano or a real Hammond. A software is just a sound source; an inhuman sound. An instrument is played and influenced by a human, and conversely the instrument also imparts an influence on the player with some tactile and aural feedback, affecting the performance. It's an instrument, not simply a sound. Plain and simple. What's that worth? I guess it depends on whether you are a musican or not.
In conclusion, while the software is OK, the M-4000D is a quantum leap in terms of sounding like a real Tron. It's why I also have a MiniMoog XL AND an original MiniMoog D. The software doesn't come close and is unreliable, especially for musicians who actually play, and play live. :-)