TLDR;
Having a PAR meter is useful if you plan on being in the hobby for many years, no matter which light source you prefer.
I could argue it is even more important for the legacy technologies like MH or T-5's over LED technology.
This is the way...
LED's lifetime is measured differently than any other sources.
As an example - look at a standard old incandescent lamp - like we all had before the LED became useful.
If you look up the life time of this lamp, it would say something like 750 hours.
But, how they measured that was really stupid and simple.
They plugged in 100 of the same lamp in a room and waited until 50% of them went out and BOOM - that was the lifetime and they published that.
So, essentially if you bought one it was a 50/50 shot on whether it would die before or after that same mark.
There is also the depreciation factor (which is what is being said with checking your LED's periodically/monitoring).
With LED's we used a .92 factor for depreciation in our calculations. Meaning a LED will depreciate about 8% over 5-ish years. This was years ago, they are more efficient today. For comparison, a MH was .68, electronic version was a .75, and a T-5 was a .85.
Now they measure LED life in L50 and L70 - which is Lifetime @ 50,000 hours and @ 70,000 hours respectively.
So far I have noticed that the manufacturers of LED lighting for Marine applications do not publish the L70 data. This is because in order to do that, you have to run a HALT test on the entire fixture (not just the LED in free air).
A HALT machine is super expensive and is the size of a car. HALT stands for Highly Accelerated Lifetime Test. It simulated a real-world environment (heat, cold, rain, salt in the air, etc.) in order to predict the lifetime of a fixture. With L70 hours in the 800k + range we have to use a simulated real environment test as waiting for 95 years is not an option.
We can get the lifetime of just the LED outside a fixture, but that changes when it is put into a fixture. Heat is the biggest enemy of a LED (much like a fat man) but it is the heat at the "joint" not really ambient heat. The joint is the spot where the LED is soldered into the board it rests on. In commercial lighting fixtures we use heat-sinks to move that joint heat away from the board. Some of the "lesser" manufacturers use a fan, which is not a good solution as the lifetime of a fan is less than the lifetime of a fixture or driver. If the fan dies and it's not replaced the joint temperature as well as the ambient heat in the fixture rises and the fixture fails.
A LED will never "die" realistically, it will just depreciate until we can't see it anymore with the naked eye.
Sorry for the long post, but this was the longest way for me to say - having a PAR meter is useful if you plan on being in the hobby for many years, no matter which light source you prefer.
I could argue it is even more important for the legacy technologies like MH or T-5's over LED technology.