Does sauteing vegetables make sense if they're in a soup?
Since fat can get hotter than the boiling temperature of water, sauteed vegetables tend to cook faster. I think the saute helps onions more than most vegetables. Sometimes I'll do that quick 'sautee' in the microwave - put oil on the onions, cover, and zap.
Those vegetables can be sauteed till they start to brown which develops flavors that you won't get with boiling. But most soup saute instructions just say 'till softened'.
Agree with Pti - also note you are really "sweating" the flavour to intensify it, rather than cooking, so you do this on a medium heat with the pan lid on to retain all the flavours for the stock - and you don't want to brown or colour the vegetables. You don't sauté in an open pan like you would to colour ingredients when making a casserole.
I disagree with PaulJ as you want to cook slower rather than faster as its about flavour extraction. And you rarely want to brown vegetables for stocks (the base of soups) as you want the pure vegetable flavour rather than caramelisation.