Here is an alternate view of the breakdown on both the S&P and Russell side.
On the S&P side, the S&P TMI is made up of approximately 3800 stocks. Outside of owning a TMI index ETF, if an investor were to try to recreate the exposure, they would start by using the S&P 500 (large cap), S&P 400 (mid cap) and S&P 600 (small cap). However, from that point, there is not an ETF that would capture exposure from the remaining ~2300 stocks. Therefore, instead of using the S&P 400 and S&P 600, an investor could use the Completion ETF (plus the S&P 500) to capture broad exposure.
On the Russell side, it is a bit different in that an investor could utilize a Russell 3000 ETF to capture broad exposure (putting aside the micro-cap differences as noted above). This exposure can also be recreated using a simple combination of a Russell 1000 and Russell 2000 ETF. To get a more large/mid/small breakdown, an investor could use a large-cap name in combination with a Russell Midcap (bottom 800 of the Russell 1000), and the Russell 2000. However, there is not an ETF that focuses solely on the Russell Top 200 (to fill in the difference between the Russell 1000 and Russell Midcap). Therefore, if an investor would instead use an S&P 500 name in lieu of a Russell Top 200, they would be doubling up exposure on approximately 300 names (the top of the Russell Midcap).
Both of these examples bring me back to the original statement in that if an investor wants to supplement an existing S&P 500 exposure to capture broad market exposure, instead of slicing and dicing their way through it, a completion ETF could be an option. Additionally, as noted in the table, it is a relatively inexpensive option, as both ETFs cost only 10bp, which may be cheaper than trying to recreate the exposure depending on what ETFs are utilized to do so.