This is me being clever. Grinders have a high-stress existence, where they are often pressured to work long hours and produce a large volume of material.
Why is it good to be a grinder?
Job security. If you're good at what you do, and you can make yourself indispensable to another lawyer, you will always have a job.
In-the-trenches knowledge. If you always work in research and writing, you will never be too far from the actual law. If you enjoy law for the sake of law, this is a great job.
Why is it bad to be a grinder?
It's hard. Law is not easy, and writing and research are not very easy things to do. It may be that you're not naturally extremely talented at them.
There's a ceiling on your salary. Ultimately, if you aren't the one bringing in business, you are providing a skill and being paid for it. There is a limit to how much you will be paid, unless you work in a very specialized area, because there is a lot of supply to meet demand.