In terms of combined arms, I tried playing the remake of the original and realised quickly that all my humans were just run over by tanks, even in a mixed group.
In RA1-2, they are good as fodder to soak up Tesla Coil hits and tank rounds so your armor can break through, especially agianst a human player. A good amount of fodder will force a human player micro against that attack, and distract them from other things.
For the most part - they are good for delay, tripwires, anti-air and scouting.