I agree, 70-90 isn't really that high (I've seen people go to 120). This is especially true if you're booming. Why have 5 workers tackling one group of nova when you can have 15 workers working on 3? Do the same with ore and you're looking at 30 workers. Then if you use the 2:1 ratio (2 carbon/food per 1 nova/ore) then you're looking at 30 on food and 30 on carbon... 90 total. Granted if you're playing on one of the smaller maps, you may not want to build so many since there aren't as many resources. And, of course, it doesn't hurt to have numerous workers building when you're looking at buildings that take a while to make--like turrets and fortresses. Btw, I consider utility boats as a class of workers, since they can repair, gather food, build beacons and (if you're playing gungans) build prefabs.
Point is that if you only have 30-40 workers then yeah, you may have a lot more only slots for military units. But with so few gathering, you're going to be using your resources up rather quickly. Course you could amass a fair sized armour of troopers. But said troopers will get slaughtered when they're up against someone with a much better economy who can afford to pump out the same amount of mechs as you can troopers...