You have a million things to do and only 16 hours in a day to do them in. Likely you already work nights and sometimes weekends to keep your infrastructure secure and running efficiently. You don’t have a lot of time to waste.
That said, think about the time you could save by doing this assessment. How many hours of meetings will you save trying to do this same exact process internally over many years? How many hours could you save in not having to deal with even the smallest data breach? How many hours could your team save by reduced trouble tickets and improved patching efficiency as a result of improving your endpoint management?
When you really think about it, though the price of doing the assessment could cost a few hours of your time, the cost of not doing it could be far greater.