Yeah there will be a functional difference, but very unlikely this difference will be in color, rather the durability of the paint. Allow me to explain;
@RebelAir gave a great definition above - 'main difference is opaque paints refract light, transparent don't, they allow light to pass through'.
If a paint is pigment based it is opaque - it will refract light.
If a paint is dye based it is transparent - the light will pass through.
Pigment is lightfast, dye is not - it will fade over time. It's impossible to have completely lightfast dye.
This means that if paint companies use dye based paints, then they won't be suitable for permanent artworks, which is obviously not what people want. Instead airbrush colors are pigment based, thus meaning that they are all opaque (I haven't used every airbrush brand, nor could I name all of them, but some major brands i'm speaking of are 'liquitex colors, createx illustration, E'tac efx, com-art, holbein aeroflash, fw inks' are all pigment based).
The reason some are called transparent and some are called semi-opaque is in fact only due to the amount of pigment in the color. Thus if you buy Createx's opaque versions of paint and then their transparent base, then no, you won't need the transparent version of the color. It is the exact same pigment as the transparent version, but with more binder.
So why do they call it transparent?
Because it's acts like a transparent due to the lower amount of pigment. The disadvantage is that the pigment WILL cover blacks and you'll never be able to create a black by spraying layer over layer with pigment based transparents as you can with a dye.
If I already have an opaque violet color, and I over-reduce it, does that achieve the same effect as just using a transparent violet (while also saving me from having to buy another bottle of paint)?
You will get the same color, but you should instead add binder or transparent base as only adding reducer will break down your paint's binder. I would recommend you look to see if your chosen paint brand creates a transparent base for their paints as just adding reducer or water will break down the functionality of the paint i.e. it's ability to stick to a surface.