Making tri-lam ELBs (my speciality) allows me to make more bows with less wood. The glue lines do not add stiffness per say. Remember, wood which is twice as wide is twice as strong, but wood which is twice as thick is eight times as strong.
The glue I use (TB3) happens at the molecular level. A good glue line shouldn't be a line at all... it should just look like the wood is touching each other. Good glue lines are all about proper wood surface prep, and consistent clamping pressure.
Gluing multiple layers mostly allows me to optimize the wood combinations and thicknesses. Plus, it provides the bonus of looking nice using wood colors with contrast. What mikekeswick has said above is very correct. Maple is excellent bow wood... one of my favourites. I do tend to use belly type compressive woods for my cores, but I have also used tension strong woods like bamboo as well.
There is also some physics going on at glue lines, which I don't fully understand. Right at the glue line itself is a neutral plane, where there is neither tension nor compression forces. If you have a tri-lam, there are two neutral planes, and I'm not sure if this is better than one with a backed bow. Something to research, I suppose. I have spoken with and spent time with some very knowledgeable bowyers like Pip Bickerstaffe in the UK, and there must be something to his willingness to make nothing but multiple layered ELBs, otherwise he wouldn't bother with the extra work.