The devices to which you refer are vortex generators. The extended nacelles, at high alpha, cause early separation in the wing/nacelle region. This results in stall values which are not music to the accountants' ears - stall speed dictates V2 speed which dictates TODR and Vref for LDR - and the accountants have a wonderful desire to flog aeroplanes - hence the aerodynamicists install VGs to delay separation and lower the stall speed, etc. They do this by trailing a quite intense vortex which comes over the leading edge and down along the upper surface of the wing.
Something in the same sort of way that military fighters such as the F/A 18 have a leading edge extension to permit very high alpha manoeuvring by having intense vortex flow over the fins.
The main concern is despatch with ONE VG missing. I recall a significant PacRim operator some years ago which did just that on a 733 as I recall - presuming that this was an MEL option. They ran around the network for a number of days before the regulator got wind of it - and the airworthiness guys got quite aggro.
[This message has been edited by john_tullamarine (edited 17 May 2001).]