Bricks, Masonry, Asphalt and Concrete - Basement Wall Height ?

I met with a builder today and he said that if it was his house (concrete block walls on footer) then he would just build 8' high wall and set the roof trusses right on top of the interior walls for support.
I have google searched several pages and cant find an answer to have 8' or 10' high walls.
If I go with 8' walls and the trusses are supported from the interior walls then don't I need thicker concrete below those supporting walls (aprox. 4" to 5" thick slab) ?
What he said makes sense and would be cheaper...I think. I mean the duct work, water, and electrical is all going to be run through and laying on the trusses anyway.
So why would I need a 10' high block wall ?
Please explain to me any pros/cons.
I'm going to call him back tomorrow morning and tell him he has the job but I'm going to have to know my correct block wall height ( 8' or 10') before they start.
Thanks

Sounds like you have web trusses? What I've usually seen is that an HVAC plenum runs the length of the house BELOW the trusses, while the ductwork is actually run inside or between the floor trusses. If that's the case in your house, and you have 8' ceilings, be prepared to have a boxed in plenum, where the headroom is only maybe 7'. 10' basement walls sure make basements feel roomier. 8' basements suck if you are hanging a drop ceiling. If it was my house, I'd opt for the deeper basement. Cement blocks are cheap, and to me, the roomy feeling would be well worth it.

Thanks XSleeper.
I know its not typical but there will not be any floor joists.
This will be a one level, block walls, house with slab floor.
Thats why I was wondering if it would be alright to run ductwork, water, elec. in the trusses.
Will I need special trusses ?
Will I need thicker concrete in the pad below the interior support walls ?
I guess I wont need a 10' high block wall since there is no support beam for a floor joist system...right ?
Thanks for your help.

You need to run your plans by your local building inspector and get any required permits and inspections. This is extremely important when it comes to safety and resale. If licensed contractors are not required in your area or if this is DIY, plumbing and electrical tend to require permits and inspections.

10' ceilings would give you some room to play. Most interior ceiling heights are 8'. If the wall is 8' and you plan on installing some type of ceiling, then you will end up with less than 8' ceiling height. If you plan on recessed lighting or anything else that runs in the ceiling cavity, you will need access. Don't cut corners. A little extra space tends to be welcome, especially in a basement where duct work, dropped ceilings, vents, and recessed lighting is used. Higher ceilings in basements make basement spaces more roomy and airy. Lower ceilings tend to make one feel cramped and feel too 'basementy.' If you are finishing a basement, higher basement ceilings make the area feel more like part of the home and living space, not an after thought or a dungeon room. Basement areas, if finished properly, can add lovely living space and up the value on homes.

Thanks Twelvepole.
Sorry I didn't explain but this will be our living quarters.
It seems that after further research that 8' high walls will be alright for our application.
Thanks