Great to hear you are enjoying your TS experience.
To summarise some of my feelings on this matter (and I imagine, the feeling of most pro players on here), there ARE indeed quite a few 'useless' units in TS. As you rightly point out, when we say 'useless' here, we mean 'useless in a competitive multiplayer setting'. The reason is normally because they are not very cost effective. Either they cost too much, they have relatively weak attack/defense, or there is another unit that does the same job a whole lot better.
A good example would be the cyborg. In most circumstances, you're better spending the money on infantry. When we move up to the wolverine and attack buggy, they are not completely without use. However, when you are playing competitively, the advantages those units bring are so limited that you are better off spending the money on something more useful. Normally a JJ (GDI), or maybe an orca/harpy if it's scouting your are trying to do.