The ODSTs are shock troops capable of realy fast (and loud) depoyment, in a real army thay would be one special forces troop out of a handfull, each specalised for different missions.
For example you wouldn't use ODSTs for a covert assination behind enemy lines you would need a quieter soldier like those in the SAS. For the games the marines and the ODSTs can cover it but if the UNSC was more "true to life" they would have around 5 specal forces and around 10 corps, proably more.
To use an in game example: I think that rather than marines, commandos would have secured the chief in the first mission. they would have had to quickly and quietly move through dense jungle so as to beat the covanant to the chiefs crash site, without attracting any attention but if things went pear shaped thay would have the fire power to deal with it. this is the exact mision thay would be traned for.
I'm not saying the ODSTs are inadequet, far from it, but thay aren't known for their Subtlety and grace.