I would have to agree with the Installer on this one, although I can't see what you exactly have for carpet and how well the job was done. But, in short, the shorter the ply, the more noticible the seams are.
What you might want to do is 'look around' at other businesses with similiar carpeting and try and find the seams. If they're not as obvious as yours (assuming the carpeting is similiar to yours), then you'll have your answer. You see, with typical household carpeting, blending is much easier. Also, the lighter the carpet the harder it is to 'hide' the seams. (Darker carpet hides best because, well, it's dark.) You could have another pro come in to evaluate the job but it's unlikely that he's gonna 'slam' a fellow tradesman unless he's from WAY out of the county.
Maybe others have better news. My best to ya and hope this helps.