In my opinion, making the missions easier is not a good reason to plan a digger's six extra points.
Dexterity is a waste. If you want to fight, just use the weapons for your level, regardless of your skill. If a digger can't beat a mission at his current level, he can just try again next level. By level 50ish he'll be able to do all 53 King's missions with 1 dex.
Quickness is a waste, similar to Dexterity. If the mission monsters are too fast for your 1 quickness digger (and many definitely will be), level up a few times, and it won't matter in the least that they hit you four times before you get a second hit in because they won't be able to hurt you much. A higher level character beats pretty much everything a lower level character or monster can throw at it.
Strength is a waste (if you're going to add pack capacity, do it with Persistence and get the extra hit points).
Persistence seems good initially for the pack space and the hit points, but consider the following points. The pack space can be earned in the game via antigrav thingies (60 thingies gives the same amount of pack space as 6 attribute points, and are WAY easier to acquire). The hit points are irrelevant to a non-fighting character for the reasons noted above for avoiding Dexterity and Quickness.
Extra dexterity, strength, quickness or hit points from persistence won't matter in the long run, cause they won't be enough to make you a good fighter and you'll be able to beat all the missions anyhow.
So that just leaves Intelligence... which also seems pretty much useless. An IQ of 1 will get you ~3.5 manapoints per level... not much, but more than enough to have the usefulness of the heal and homeport spells before too long. An IQ of 6 will get you about enough manapoints to meet the mage tower building requirements for your level (10 manapoints per item level), and considerably more than enough when you drink a DD in the pub. As for what you would need or want to build in the mage tower with your digger, I have no idea. Maybe you want to build composiums out of the stuff you dug up. Maybe you want your digger to be building items for your other characters to save them clicks. I dunno.
So to sum up, they're all useless. The least useless are clearly Persistence and Intelligence for the hit points and mana points. Personally, I went with Intelligence for my four pure diggers and remain happy with that choice. Although, I have yet to use the mana other than casting healing spells while doing the missions.
Sandy