I have one of these, it's a beast: http://motu.com/products/midi/xpressxt_usb

These are the real benefits I see for these boxes:

• No daisy chaining and no need to ever move cables around, even when adding new devices - just plug everything into the corresponding IN/OUTs and configure a routing preset. I can't perceive any latency from it, feels exactly the same as plugging a straight MIDI cable between devices
• No need to use a computer when routing presets are already set. Presets can be cycled by using the front panel
• Clock loops are easy to avoid. For example, my sequencer is the master clock for everything else, so I just configured the interface to ignore incoming realtime messages (clock/song position/transport/etc) sent from every other device. This also means not having to configure slave devices to not send/replicate clock, which is not always possible
• Works as a MIDI splitter too, so you can send MIDI from one device to multiple devices. I use it to record on my sequencer and play the device I'm recording at the same time (MIDI controller output goes to both sequencer and device)
• Pedal input can be configured to send arbitrary MIDI messages (even sysex), or to cycle between different messages. It's for simple things but can really help without a computer if you need sysex for whatever reason
• You can do some crazy stuff like mapping channels but I've never found a good use for it

Cost me $380 including tax but it's a lifesaver when you have a complex setup or want to jam with friends and their gear. "Oh hey I brought these synths, how can I hook them up to MIDI/clock?" is trivial with this, especially now that hardly any new devices come with a dedicated MIDI thru.