Here are things to consider:
Experienced builders don't care about the takeoffs on a big picture basis, the takeoffs are usually wrong, even if perfectly done. In our experience half of drawings we receive, are heavily revised by the order is approved (heavily revised meaning over 10% has changed). EWP, structural metal need to be accurate but framing lumber, and sheet good can be off on counts at the lift quantity (+-1 lift for an average house).
Suppliers aren't responsible for the takeoff so the amount the quote is negligible (see drawing revisions, and trades can misallocate the materials - This can't be reasonably traced). Over? The customer ends up paying less, under? The customer pays more. This has been universal where I am (Ontario, Canada).
A large minority of plans are missing key elements (like sheer walls), pointing out, and showing these differences would be a big value add for the consumer (contractors using the materials) by the supplier.
Good customers understand that lumber is a commodity, a lower price this week can flip next week, and they'll contact their preferred vendor about the differences.
There's always a preferred vendor.
Not great customers will shoot drawing off to multiple suppliers, causing them all to do the same takeoff, wasting time, and money, only to deal with the same issues above. They'll still go back to their preferred vendor to get the lowest price.
Summary of the above is: EWP, and structural metal are key items because they rarely change, framing lumber, and sheathing requirements change all the time. What you're looking at is helping suppliers capture the bad customers (which are often the biggest, to be clear), but saving suppliers the time handling them is great. Also, accuracy, and pricing isn't that important (with caveats).
This isn't a statistically significant sample size, consider it anecdotal.
As a side point - sometimes the shady lumberyards do bid too low on purpose to win business. Then later have the contractor submit a change order. This often hurts their reputation unless the contractor is in on it to win a bid. The supplier doesn’t tend to lose money though as the bid is for the quantity of materials.
You could turn this into a selling point. As in, helping a contractor or competing supplier verify the takeoff.
I agree with your point, having a second, impartial source is important to confirm the ballpark.