Looking at it purely from a financial incentive side of things, do regular season ticket holders provide as much monetary value to club?
They get their seats for less than it would generate the club if they sold that seat to 19 different people on an individual basis over the season. ST holders are not very likely to be handing over money in the souvenir shop every week, as a tourist just there for one game might do. And if it's a different tourist every single week in that seat, with each trip from each tourist a 'destination' spending experience, that financial gap grows furter.
Someone going to Old Trafford for the first time is going to hand over far more cash than someone who has been going for 20 years and doesn't want to buy everyone in his family a gift celebrating the day he went to see United
Plus the limited availability of away tickets and the inflated sell-on price. It's not unexpected a club will look at that and ask if Tony, ST holder for 20 years, can get a couple of hundred quid for a seat in the away section re-selling it to Bob, why can't the club just sell that ticket individually to Bob and make more than they did sellling it to Tony?
I think it would be a mistake as there are other factors at play. But when you're still in debt and looking to finance a stadium as well as try to compete on the field, nobody should be surprised.
A non-regular match goer will stock up on merchandise and maybe even treat himself and his kids to some hospitality. From a pure financial point of view, it is clear that type of match day ticket holder is more valuable each week than someome who gets bus to the ground as he had for 30 years, watches the match, leaves and is having his tea back home 45 minutes later.