Kevin, KF7MYK, provides a great example of what happens when operators do not use phonetics. You may think you are saving time by not using them, but the fact is that unless someone is familiar with your callsign, they may copy it incorrectly. You want everyone on the bird to have your callsign correct so they can call you with the right call rather than having to ask for it again in phonetics, or worse yet, getting it wrong.
If you call someone using just their suffix, it means that you don't have their prefix. When you hear someone call you that way, you know they need the missing part of your call. If you have someone's complete callsign, don't call them by their suffix. That's not their callsign. Sometimes you only hear the grid square and call someone that way. This is an indicator that you need the persons complete callsign. A complete exchange is what we should strive for.
If you call someone with an incorrect call, other stations pick up on that callsign and the inaccuracy propagates throughout the pass. You find yourself looking them up on qrz.com and not finding a match. Then you are listening to the recording and trying to figure out where you went wrong.
Satellites have varying audio quality and the users radios are all over the place in terms of level and clarity. Phonetics will get the callsign through correctly and avoid the problems associated with not getting it right. Everyone is not copying the bird full quieting. Give them half a chance.
Also keep in mind that when you are working someone whose first language is not English, they may have even more trouble with your callsign if you don't use proper phonetics. Many non-English speaking amateurs know just enough to copy a callsign, give you their name, report and QTH. They learn the International Phonetic Alphabet and will recognize the words associated with the letters. If you use some other words, they may not understand what letters you are trying to convey. Adjust your operating based on who you are trying to work.
Remember that everyone on the bird is listening to your QSO and hopefully writing down your callsign for future reference. Use phonetics and SLOW DOWN. Don't run your grid square into the end of your callsign. Announce your callsign at a constant pace. This methodology will help everyone get it right the first time and save airtime in the long run.
All this, of course, in my humble opinion.
73, John K8YSE