Most modern industrial parks only have a few industries. Of those, typically the same one or two are consistently worked on a weekly basis.
Take a golden retriever or German shepherd to the park and fling a tennis ball across the field and you will have one ecstatic dog. They’ll chase that slobbery sphere endlessly until they’re to the point of dropping from exhaustion. Take them tomorrow and they’ll be just as thrilled. The day after that, same thing. Happy. Over and over and over, they never get tired of it.
When it comes to my personal operating sessions I’m basically like that retriever. I can run the same scenario day after day and not get tired of it. I’m easily entertained I guess. Simpleton or not, it is pretty prototypical. By and large prototype railroads do the same thing week in and week out in terms of the work they do. A typical branch has one or two large customers that account for most of the car movements. Sprinkled in may be a few more that get a car every quarter, six months or maybe just once a year.
Be that as it may, the amount of operational variety you need to feel entertained has a direct impact on the design of your layout. The more industries you need to work in an op. session in order to have fun, the closer the structures will need to be spaced. You may also have to make the structures smaller in order to make room for them. You’ll lose a sense of realism which may or may not matter to you. As I always say, be selfish, do what brings you the most enjoyment. However, it’s always worth asking yourself if you are indeed going to work those additional industries if you put them on the layout? What’s the visual damage of “over stuffing” your scenes? Do you care? Is it worth the tradeoff? With any design approach just make sure it’s a well thought out decision where you’re clear on the pros and cons and not something you do by default or out of an irrational fear that you’ll be bored with the layout.