Road shoes vs mountain bike shoes

I own and use both types of shoes regularly.  Mountain shoes do have a recessed cleat.  They are easier to walk/run in than road shoes, (feels like walking in baseball spikes... the plastic kind) and you are less likely to have trouble clipping in if you get mud or debris in the cleat.  Mountain cleats are double sided so, for novice riders not used to clipless systems, they are probably a little easier to learn on because you don't have to fumble around as much to find the pedal cleat and flip it into position.  

I have just mountian bike shoes now (after my last tri bike and tri shoes were stolen along with my car), mainly so I can wear them on both my tri and mountain bikes and I didn't have to buy two new pairs of shoes at the time.

That was over two years ago and I haven't really seen to need (for me) to purchase another pair of tri shoes.  Mine are velcro, so they are quick and easy to put on.  They are super easy to run in (I haven't perfected the flying mount/dismount yet).  I wore them when I did the half-Rev at Cedar Point in 2011 and I thought they were just fine.

I have never had any issues with my road shoes in transition. I would not like to do a tri in my MTB shoes, nor do I see the need to.

I run spds on all my bikes, though, so YMMV.

Also, not all mountain shoes have recessed cleats.  I just had on my pair of commuter sandals with mountain SPD cleats and I am walking directly on the cleat itself.  It's slippery and noisy. 

I like mountain shoes for around town, commuting and anywhere I'd walk some.

I use TRI shoes (not road shoes) for racing.  They are vented, easy to slip on, don't require socks, and have a single quick velcro closure.

I've actually never owned a pair of "road" shoes

With Road shoes you won't run out of T2 and halfway down the block high fiving people and thinking, "wow this is going to be a tough run, good thing that transition was so fast",  before realizing you still have your Mountain bike shoes on...

     don't ask.