1) They've already got quite a lot of game, and there are still 10 months before Skyrim is released.
2) New engine* - any issues caused or worsened by Gamebryo have probably been addressed early in development.
3) Fallout: New Vegas had quite a few problems with bugs on release** - Bethesda are probably fully aware of the potential for negative publicity.
4) I'm an evil PC/mod user and I can hopefully fix most minor bugs myself.
5) I haven't had any serious problems with bugs in Morrowind or Oblivion.
*I know many bugs have nothing to do with Gambryo.
**I know Obsidian developed it, but Bethesda published it and most people don't know the difference between Bethesda Softworks and Bethesda Game Studios.
Fallout:NVs bad publicity from the bugs saw it dropped from like £50 to £20 in most stores like HMV where I'm from and I'm sure Bethesda doesn't want that to happen to Skyrim.