There are six levels of troop experience in SMAC, called morale levels.
Very Green
Green
Regular (?)
Hardened
Commando
Elite
(Oops. maybe I got a bit confused with CivIII-terminology) I guess you can find the real list in the datalinks.
Elite troops get +1 movement.
A unit might get promoted, if it wins a battle.
It will get a higher level of morale from start by building certain base facilities like:
Childrens Creche
Command Center
Bio-Optimization Center / Naval yard / Aerospace Complex
(for Land/Water/Air units respectivly)
Also there are some secret projects granting you free facilities:
Command Nexus = Command Center everywhere
Cyborg Factory = Bio Center everywhere
Maritime Control Center = Naval yards everywhere
Some social engineering settings affect the moral of your troops. (Wealth -2 /Power +2)
The Spartans get +2 morale by default.
The Gaians -1, IIRC
(Miriam gets +25% power, but only on attack.
All infantry has an attack bonus against enemy bases)
But only attack, if your units have 1 movement points left. Do not attack with troops, that haven't got one full movement point left, except when absolutely necessary.
I guess that's why you feel, foot soldiers are weaker than rovers.
If your troops visit a monolith, they might get a morale upgrade for free. But eventually, the monolith will vanish if used to often.
There is also a "trained"-option available in the units workshop, which gives +2 morale. (I forgot the tech it comes with)
Similarily, the is true for native life forms, which ignore weapons power, but only fight by morale level. The according base facilities are:
Bio-Lab
Centauri-Park
Temple of Planet
The according secret projects are:
Xenoempathy Dome
Pholus Mutagen
Build all that stuff, and you'll see your troops become much more powerful. Move your troops through fungus to raise some worms - if they survive the battle, they might get promoted.