I have always used MIDI-based controllers (Roland A-37) and synthesizers (Korg Triton Rack, etc.). I am thinking about adding a new controller, but I'm not sure if I should pursue or avoid a USB-only controller. Other than controlling other MIDI-only gear, which I probably won't do, is there any reason to require MIDI capability on a new controller? It will only be used in the home studio and be connected to the computer for DAW use.
If you won't be using it on stage or with other MIDI gear apart from a computer, a USB only on your keyboard controller should be okay. (Personally I find the connection more stable)
Modern PC's should allow you to route the MIDI signals to other USB-MIDI devices plugged into the PC within the OS or software. In my experience with Macs, you can do the routing with the Audio MIDI Setup utility to even route MIDI signals over IP.
That being said, if you have any inkling that you'd want to use your controller in a setup without a PC, get one with regular MIDI ports (or one with both).
From what I have seen MIDI is a struggling technology but has a very practical and fundamental use hence why it has stuck around so long despite it being technologically old. If I were you I would not mind getting the USB only device if you are not intending on controlling a MIDI only device anytime soon. And even if you are you can get a USB to MIDI interface (like the ones MOTU makes) and route your USB keys out through one of the interfaces ports to what ever device you may need. MIDI its self provides no fundamental advantages (IMO) over a USB device if you are strictly talking about the medium by which the keyed input gets transmitted to the DAW (assuming the DAW has input for both devices)
For what its worth USB allows much faster data transfer than midi. In most cases the USB only devices really just have a small MIDI interface inside the box so when you plug it in it just shows up as a MIDI interface although there are no MIDI jacks. Remember no computer has MIDI inputs anyway, you will always need some kind of a USB/Firewire/thunderbolt etc. to MIDI converter anyway so you are just eliminating the middle man here by buying a USB keyboard.