The thing is if you ask Mike Tuchsherer or Brad Gillingham if strongman will make your midsection stronger- they'll say sure. But if you ask them how to train, as a powerlifter, for deadlifts, I'm pretty sure they'll say deadlifts, pause deadlifts, RDLs, rack pulls, squatting etc.
Strongman covers a diverse array for events- some of which call on strength-endurance, carrying etc and a range of different movements and skills. All of those things will help develop athletic attributes that carryover to PL, but the best thing you can do for powerlifts will be built around powerlifts. What do strongmen do specifically to improve their deadlift? The same stuff Gillingham does- but obviously they do more to cover the diverse events they do. Obviously I'm saying there is some value with the notion of specificty in training. Its not just about building muscle groups.
As far as I can tell the Russians are leading the way when it comes to the science of powerlifting and optimal strength devlopment for PL. They are not sending Sorokin, Fedosienko and Barkhatov out to flip a tyre or lift an atlas stone. When Brad or Mike are preparing for a PL comp or a DL comp they are not carrying an anvil or deadlifting cars.
Here is Brad's deadlifting outline from his blog:
HMB Blogs
He does talk about using strongman stuff, but kind like assistance. Did you see my interview with Brad on my Iron Edge blog? There he said he adds in strongman stuff only to keep the training interesting.
Digging holes will strengthen your mid-section. A strong midsection is a key to a big deadlift- it doesn't follow that digging holes is a good way to improve your deadlift.
I'm not saying strongman training is not beneficial to the powerlifts- but not accepting the suggestion that strongman training is better than powerlifting training approaches for deadlift.
Back to the original question- I think plenty of good deadlifters pull a number of times per week. I know Mike T pulls heacy twice per week- although it tends to be one comp pull and one deadlift variation, like RDLs, pause deadlift or deadlift with chain.