Benefits?? Laziness, penny pinchers, saving money because you're too cheap to buy a new edition of the code, no time wasted writing new WPS's were applicable. Same old story, time and money.
Validity? Sure, they are still acceptable under most circumstances. Especially a 2008. Not that old. A lot of job specs will actually still reference the 2008 edition as it's authority as projects engineered, approved, permitted, and in the fabrication/erection process now were put on paper with the 2008 being the updated code at the time. Inspections need to be done to the same code the project was designed and approved to. Some changes could open quite a can of worms.
As well as the fact that clauses 3 and 4 don't really go through that much change from edition to edition. So all the prequalified WPS information in Clause 3 is pretty much the same as well as the information in Clause 4 on Qualifying Procedures and/or Personnel.
Now, I understand your concern that a company that is writing WPS's to cover work to be performed currently is using an older edition. WHY don't they have a new one? SEE POINT #1 (first paragraph). Revising to make an existing WPS conform to current code and/or job specifications is not as big a deal as developing a new WPS using old codes. Though, as already stated, those sections do not change much very often.
Object/Bottom line: make sure they conform with job specifications and welding manufacturer specifications on consumables. Then, make sure the new ones in particular do align with the 2010 edition if it is needed.
One question, do they have the 2010 and just forgot to change the form down at the bottom where it says which code it is too. They may just be using an old downloadable form for the WPS's and didn't know they could/should change that.
He Is In Control, Have a Great Day, Brent