The way I think about it is paying for an exceptional trainer instead of an advanced is like paying for 4 scouting points per week instead of 2. There are tangible benefits (extra pops) but you pay way more for each extra pop ($10K/week per scouting point at 4 instead of $5K per week at 2). The difference between advanced to exceptional level trainer is even worse, because an exceptional will likely have more than 2x the salary and an astronomically higher price to acquire than an advanced trainer. And the benefits are only probably 2-4 more pops per season, or maybe 18-20% over a level 4 trainer. So clearly, not financially worth it.
The only reasons you would have an exceptional trainer: (1) You are training a NT player, (2) you really love training and aren't concerned with doing everything perfectly optimally geared toward club success, or maybe (3) you are tanking and making ridiculous weekly profits anyway. Just like if you know you will have the #1 pick and have only a few weeks to interview several intriguing prospects, you may find it temporarily worth it to invest $40K/week in the draft for the extra 2 scouting points. Long-term and for most teams though, this strategy doesn't make optimal financial sense.