Comment by lumost
The concern with over-employment is that many "healthy" organizations rely on trust. Someone says it takes ~4 weeks to do something, I don't want to have someone else "re-scope" the effort to verify that it really takes 4 weeks. If someone is only doing 3 days of work each week - then realistically this task could have been done in a little over 2 weeks.
On a long enough time horizon, someone will pick up on this and perceive the engineer as "slow." If multiple people are doing this in the team - then the org is probably in trouble.