You don't need seedling. A vigorous clonal rootstock would work just as well. It all depends on how big you want your espalier. Many of the spur varieties are low vigor thus the need for a more vigorous root than one might want elsewhere.

You may not "need" a seedling, but if that's what the espalier folks are doing...why not? If using seedling rootstock for espalier spur varieties is the generally recommended method I wouldn't use anything else. It would give you an opportunity to "build" your project from the ground up and could be done completely without any monetary cost inputs from you.
You can grow your own seedling easily within 1 year and graft the next or even bud in the summer. You can also purchase named vigorous seedling rootstocks with resistances and robustness not typically found in other stock.

Seedling rootstock is very rarely used. I think you are referring to the use of seeding rootstock on the low vigor varieties like StarkSpur Red Delicious. StakSpur trees are low vigor mutations of tip bearing VarietyâÂÂs. Most spur bearing varieties have equal vigor with tip bearers.
You do want spur bearing verities for espalier and a rootstock 30-40% of seedling like M26 or B-9
Good luck.

Orchardace...don't you think a slow growing variety like Goldrush might do very well espaliered on seedling stock?

I grafted two small pieces of GR last summer to a seedling stock (I grew from seed) and they put on more total growth than the established Goldrush tree I have on M26.

I haven't tried espalier before, but if it were me I'd want to get things underway. I dislike trees that seem to want to remain tiny "plants" forever.
I would think the increased vigor of seedling stock would be a welcome advantage when growing slow varieties especially when espalier is the plan.
Again...I've never done it so I'm just thinking out loud. (??)

I just finished planting two acres of espalier trees. I did try to research the well but, time will tell how my spacing and choice of rootstock worked. You could espalier a seedling tree. I'm guessing the downside would be waiting longer for fruit, spacing them 20 to 30 feet apart and having to prune more often. The only advantage i could see would be not having to water. My strategy was to adjust spacing depending on vigor. I spaced trees 8' to 14' apart using m26. On a six foot trellis. I espaliered a few trees on m111 before I researched the issue a bit more.

I see...so most folks are doing espalier on rootstocks mainly dwarfing stocks.
I probably should do an espalier apple or something. My spacial constraints, and orientation would be ideal for it.

I do like the neat and tidy look of it, but I think I'd try the seedling stock anyhow just to speed things along. The prolonged period until fruiting would be a key disadvantage though as you said.
One additional benefit however would be longevity and possibly ultimate bearing capability.