TES games tend to be huge. For a huge game like this I suspect they've been working on it for a very long time. Maybe starting with the concept around or before Oblivion was released.
I'd guess that adding Spears would take more than a month. Keep in mind that right now the game should essentially be done. Hopefully they are just doing some final testing, and making the arrangements to print and ship.
To make any change now would likely disrupt the schedule more than a month.
I suspect that mod'ers will add Spears if it is allowed by the engine. And if it's not, it would be tough for Bethesda to add, because they'd likely be unwilling to mod the engine at this late date.
If it's supported by the engineer, Bethesda could release Spears (and other things) via a DLC or something. I'd rather that than a month delay.
There will always be cuts do to developement time. If Bethesda softworks were to stop time, would they make this game a lot better? Of course, but they can't stop time. Always room for improvement buddy.
It's not just time. It's also money. And time is money - close to literally. Since Bethesda would then need to pay the developers for another month. And even if the developers where free, that would be a month that they would not be working on some other Bethesda game or DLC.