I bought a used road bike last year that came with Ultegra pedals. I already had mountain bike shoes and pedals, which are what I'm currently using. Would it make sense for me to buy road shoes if I already have the pedals? I know there is a pretty big weight difference between the shoes/ pedals, and the platform is bigger, but is it that much of a difference that I would notice it?

I would say stick with one system unless you want to invest in a new set of shoes, which does cost money. I use mountain/bmx shoes with spd pedals on everything since I don't see a lot need for two systems.

I have two systems but only use my SPD/MTB shoes. I love my MTB shoes, te carbon soles are stiffer than my roads with carbon sole too.
My road shoes are 3 straps, & don't really match my bikes, so I use them on my trainer. I like having the ratcheting strap to keep everything in place.
I may get a pair of road/SPD-SL shoes for next season, or end of the season blow out.

yes, try giro's if you have high arches, freeking amazing shoe.

I have both road and MTB pedals and shoes since I ride both road and mountain bikes. Road shoes are awesome when you're on the bike and clipped in and suck when you're not. I like the feel or the big platform but my single sided Look pedals require a little more work to get into since you have to flip it to the correct side. If I have to any walking at all, I use rubber covers to cover the plastic Look cleat which is easily damaged. With no heel, no tread and a hard plastic bottom they can be very slick if you have to walk on concrete. I've had a few interesting moments on my painted garage floor. If I'm honest about it, I mainly use the road shoes to avoid roadies from looking down their nose at me which they probably will do anyway since I prefer my Camelback to water bottles. I am a mountain biker at heart. For touring, where more walking is required, and city riding, more clipping in and out, I will put the Crank Brothers MTB pedals on. I have a set with a full platform allowing me to ride without cleats if the mood strikes me. Translation, flip flops to the pool. I've gotten to the point where I don't really care if some roadie snickers at my setup, especially if I'm passing them. I can enjoy being a "Fred" on the road since I know those road bikers would totally freak out if they had to put their bike up a 3 foot ledge. Do what feels good to you! ----- Johnny P from the Be Self Propelled Blog.

I've used mtb SPDs for many years. I also have Look type road pedals. The SPDs are easier to clip in, and just as lightweight. You can get good lightweight road shoes with super stiff soles that take SPD cleats, but walking on the little steel cleats is problematic and really rough on floors. The big plastic Look cleats are easier to walk on, generally won't damage floors, but still are nothing you could walk any distance on. They also wear out pretty quickly from walking on pavement or concrete. Another big difference is that with hard-soled road shoes, you can't really put any force on the pedals without clipping in because your foot can very easily slip off the pedals. With mtb shoes you can tromp on the pedal and the rubber soles will help prevent your foot from slipping off the pedal. This is very useful when getting started quickly after stopping for cross traffic. A few strokes will get you quickly going across the road and then you can clip in.

With stiff soled shoes, the soles form the platform for the foot and I can detect no difference between the mtb pedals and wider Look pedals.

I feel mtb SPDs or Eggbeaters with good quality, light, stiff MTB or touring shoes are much more generally practical. The only thing you're sacrificing IMO is the somewhat greater weight of the shoes.