Yes, it depends on what you want from the trousersWould builders trousers be ok with built in pads?
It's still not essential to get paintball specific items, depending on what you want from your gearI would stick with the purpose built paintball playing gear. If cost is a consideration (when isn't it), then look on the "For Sale" forum page, and purchase them second hand. There are bargains to be had, just wash them before use.
I have found my elbow pads and knee pads promote paintballs bouncing off me, not bruising, and keeping me in the game longer. The knee pads are awesome, as it makes kneeling down really comfortable, when hiding behind those oil barrels.
Correct regarding our pants padding, there designed to be used with knee padsI used to wear no pads and UK army issue pants, this was very uncomfortable in the woods. Then I wore dye knee pads under the army pants and it was a lot better as there was no issue of roots/gravel etc cutting up my knee. I recently upgraded to wearing PE playing pants over my knee pads and its so much better. The padding is slightly too thin to not wear knee pads in my opinion as its more for bounces/light sliding.