It's not important. What is important the strength of all muscles for the intended purpose of those muscles. Transverse abdominis is a core stabilizer of the lumbar spine and a strong core doesn't only support the spine, but also result in more power available through the extremities.
A lot of research has been focused on attaching control of the transverse abdominis with lower rates of low back pain and then a lot of time can be spent in isolated exercise, but at the risk of missing the bigger picture which involves much more than one muscle.
So...is the transverse abdominis important? Yes, very much so!
Should you spend time and effort to isolate it? Not unless you have a lot of extra time on hand and nothing else better to do. If you want to make the most out of time available to exercise to help prevent low back pain - get into a cardio exercise routine. The effect on back pain is the same and you have the fitness benefit as a bonus!
In general, common sense prevails over fads. Transverse abdominis isolation is a research fad.