Actually the VT-4 has an extensive MIDI implementation (via System Exclusive MIDI messages, though) and all its functions can be edited via MIDI.
Using MIDI you can create very different patches, ranging from natural (little autotune/EQ/Reverb) to classic-electronic (different tastes of vocoder/robot/talkbox harmonies) to extreme and out of control (feedback, synthetic sounds, glitches). It is much more flexible than you would think of from a first sight. Much wider range of effects than the VT-3 (which was only preset-based, MIDI only via USB and limited to a few MIDI CC’s only).
My guess is that Roland decided to get the price down and limit the interface to only a few knobs, so many features have gone in the background (only partially available through preset “variations” and of course MIDI). The alternative would have been a much more expensive box I suppose. The negative aspect is that unless you dig the MIDI side of things (and know how to handle MIDI System Exclusive requests and data transfer) you cannot get these sounds out of the box, which is a pity.