Oath is a strong deck, nobody is disputing that. However against workshop varients it simply rolls over and dies. Anything that can remove Oath's two creatures makes it roll over and die. Furthermore, adding white could benefit the mirror match.
If you take a mis-metagamed oath anywhere, it will have a tough time with workshop decks. If you know they are going to be there, you can EASILY deal with them with the colors in your deck. Remember, Jaco's build runs rack and ruin side, has access to artifact/enchantment kill in green, and generally doesn't have a problem with artifacts since it is well prepared.
Reasons not to add white
- Makes mana base more unstable. One of the strengths of Oath is that it can play Wastelands and Strip Mine, adding white might negatively effect the whole "mana denial" aspect of Oath.
- Oath is a tight deck, how much benefit could white truly add to it?
Another major reason not to run white is that these cards make you more succeptable to mindslaver activations (stp your creature, disenchant/seal your oath, etc). The best white card (balance) is good, but probably not worth splashing white for...
My oath build (~6-8 cards different from Jaco's) has no major problems with workshop decks, and doesn't care about mindslaver activations unless they are of the infinite variety. This is because it is properly metagamed for the decks I expect. (which includes workshop)