It's better to give them rhinos and the combat squad if you find the game favors it. It's easy to take out 5 space marines in either shooting or assault. Kill points don't favor the practice and while you can take more objectives by using combat squads you're creating very fragile place holders. Other units can get away with small space marine units a la combat squads (because they either have furious charge and feel no pain (BA), are chaos space marines who have yet to be introduced to fear (SW) or are magicians with machine guns and excalibers (GK) AND they don't need to bring a full table for the complementary dessert - I'm looking at you, 5 man BA assault squad with a melta gun. yep, 5 marines will last longer than 5 guardsmen, 5 dire avengers, 5 termagaunts, 5 firewarriors, etc., but not appreciably longer.