I've never really considered it, actually. More specifically, I've never wanted more than 2 Jace, the Mind Sculptors in my 60. OK, I did consider playing an additional 1 in my SB to help with the Peacekeeper plan since I only had 2 MD in Dreadstill. Why am I bringing this up now? I read some articles this week, lemme gimme you some examples.

"Is Jace that important? Hells yes! Have you ever cast that card? There's basically nothing that it doesn't do. You want to draw cards, you want to temporarily deal with big dudes, you want to eventually kill them, and Jace does it all singlehandedly. Do I really need to explain why decks that can cast Jace should play four, especially when it's a focal point of the deck?"

- Gerry Thompson

"Not every deck is actually a four-Jace deck, but I'm pretty sure that there are way more people using fewer Jaces than they should than people playing too many. When you can Misstep their turn-one play, Hymn them on two, and drop a Goyf on three, you're generally going to be able stick Jace on four (and defend it). Bouncing a creature is just brutal in Legacy, since far fewer creatures in this format have haste or "enters the battlefield" triggers than in Standard. It's a format of almost all Baneslayers. On top of this, it makes your discard even more valuable later. Additionally, there is a lot fewer burn spells and manlands that kill Jace in one hit. Besides, the combination of free counters, amazing discard, and land destruction make the advantage from Jace pretty insurmountable.
...
At this point, no one disagrees that Jace is a "10," so don't think I'm arguing that issue. I'm saying that Legacy is a four-Jace format, a position that's held by very few people at this point. "

Patrick Chapin

If I'm not reading this wrong I don't think they are advocating every Blue deck is auto 4-of Jace but that more decks should run 4 Jaces. Outside of Landstill/MUC/decks-that-like-to-draw-lots-of-games would you ever consider playing more than 1 or 2 Jaces?