Well, in regards to tactics, they do not matter, sadly. The dice rolling system is unable to differentiate between the millions of tactical decisions that are made on the battlefield. As for the base defences, they are an investment worth making, however the fact that they take so long to build means that in a conflict over a commonly contested territory, it may become more of a nuisance than an investment. As for how they're used, I mentioned the exact same thing to Martin Lichtman. They are a way of being able to bolster the amount of casualties you inflict upon an opponent.
When the individual who attacks a fortified location rolls the dice for their casualty percentage, they are required to then add the percentage that is shown by the type of base that is present. Which, if you're The ECA, can mean that a person has to add an extra 40% to their casualties. It goes without saying that such a large percentage could easily cause the complete destruction of your troops.
Yes, you can. The way you could perform such a feat would be to lump all of your troops together, so that a force of 5,000 SAP troops and 15,000 American troops would become 20,000. Whoever is in overall command of the force will be in charge of the dice rolling, and the casualties inflicted would be a 50/50 split between the two forces. So a 50% casualty rate in the aforementioned 20,000 from North and South America would mean both would lose 5,000 each, since 50% of 20,000 is 10,000 and a 50/50 split is 5,000 each. This does mean, sadly, that the SAP in this joint effort would be eliminated.