If you were straight in, behind other traffic then things might tighten up but then good airmanship would have you using TCAS to judge the gap
ATC are trying to do the 'judging' or 'traffic control' for us and if things do 'tighten up a bit' due to the extra speed we've decided to carry then we'll be asked to slow to min app speed sooner than necessary and BURN MORE FUEL. Of course if it's quiet it's slightly different but then ATC doesn't generally assign speeds in this case. When it's busy with traffic behind, in front and waiting to cross and depart it's slightly more complex and we are generally unaware of the picture ATC has.
3/4 of a mile does make a difference as ATC need to ensure minimum separation.... You take a lot of pressure off of yourselves when you do the speed requested. Yes controllers cope as they know we do not fly the speeds requested accurately but look how close it can get with 'expect late landing clearances' and land afters etc. In the current level of management 'support' why risk it?
NB: The image on TCAS is not an accurate indication of an aircrafts position and certainly not a primary tool to determine our own separations. Three main factors which contribute to traffic display misinterpretation:
- Nature of the TCAS moving reference display.
- Partial traffic picture given by the system.
- Limited accuracy of TCAS bearing information.
ATC however have the accurate picture sitting right in front of them... let them use it.