Could you be more specific?
It's a matter of brewer preference (esp. when it comes to step-mashing), but I believe there is more BS with using a cooler because of several reasons:
1. It’s not SS. Professionals use SS for many reasons: cleans easily, looks pretty, durable, welded fittings.
2. You can’t direct fire it.
a. Direct firing allows me to maintain temperature completely independently of my HLT. This means that I can use the HLT to prepare for a consecutive brew or preparing my sparge water without needing to worry about what the MLT is doing.
b. The most important reason to me (I’m a strong believer in step-mashing) is that direct firing allows a much easier step-mashing process because you don’t HAVE to add water and HOPE it neutralizes to the correct temp. Chasing around mashing temperatures is a PITA.
c. You have to design/build a system that won’t melt your cooler. It’s much less BS to desgn/build a 3-keggle brewstand, IMO.
3. The size and shape. I prefer a round-shaped MLT. The biggest round cooler that I could find is 10 gallons which limits my OG capabilities.
4. I noticed an immediate increase with my brewhouse efficiency when I switched to sanke. This is because of my expanded options of easier-to-perform-with-a-keggle-than-cooler brewing techniques.
If you need more reasons, just let me know. I hope I never have to use a cooler again.