It all depends on how old your magi is. 50 years past gauntlet? More?
For a mage with arts in the 20-30 range, even with a relevant focus, they really need all the extra pen they can get. Going out and risking life and limb to get an arcane connection, then spending the time to make it permanent, then building it into the magic item to kill the creature.
Would they waste a season and risk danger to get +8 pen? Or would they spend that extra season safe in their lab to make a higher level effect?
Assume Magus Alphonse has a Pen score of 5, a Creo of 20 and an Ignem of 20. MT 10, Int 4, Inv Gen +3, Phil 7, Craft 6, Aura 6, Lab bonus +3, he has a focus in fire.
Item Ball of Abysmal Flame Wand
CrIg 35 + 5 (24 uses)
CrIg lab total is Cr + Ig + MT + Int + IG + (Phil + S&M > MT) + Craft + IG + Aura + bonus + focus
20 + 20 + 10 + 4 + 3 + 10 + 6 + 6 + 3 + 20
Lab total of 102
As the base item needs a lab total of 80, we can put in 22 lev of Pen - item is now lvl 51 effect.
If we drag this over 2 seasons to add more pen the numbers change to
102 / 2 = 51. Therefore can have a lvl 51 effect over 1 season. 22 Pen
102 / 3 = 34. Therefore can have a lvl 68 effect over 2 seasons. 56 Pen
102 / 4 = 25. Therefore can have a lvl 75 effect over 3 seasons. 70 Pen
102 / 5 = 20. Therefore can have a lvl 80 effect over 4 seasons. 80 Pen
As you can see, devoting extra seasons to the project gives you diminishing returns. You spend extra seasons to get less results. If the player had their AC they would still spend 3 seasons crafting the item, but would have a pen of 81 instead of 70.
Also, the ultimate SG decision - making an AC useful for an item gives the players a reason to go out and adventure for it.