What's the problem with getting no HUD?

[Infininja]

Are people just having difficulties with the earlier levels since SP levels can't be rebuilt? If you use BOLL's lesser HUD and make yourself a Sentinel there's no HUD besides the red arrow 'hit' indicators, objectives, and nav points... so are we just trying for the earlier levels now (and Keyes, I don't think that has Sentinels)? (I know someone has done it, and he's about to do a write-up about it at HBO, but it's pretty complicated).)