I wouldn't say accusim is 'must have' as there are many people here who have different preferences. I will say, I would not be nearly as much into this hobby if there was no 'study sim' option available to us.
I have mentioned it on the A2A forums. When I fly a non accusim plane, I feel like a 'flight sim enthusiast'...which is fun and fine when I want to do that...Sometimes I do thoroughly enjoy being a 'flight sim enthusiast'. FARs are sure fun to bust aren't they?

When I fly Accusim planes, I feel like a pilot. It makes me draw on my real life training, and rely more on my instincts, experiences, and knowlege. It makes me think and do more of the things I have to think and do when I fly for real. As such it is excellent practice to keep my brain in the game, help my brain stay 'current' during the times I can't fly for real.
For instance starting a typical airplane piston engine. A real pilot must think about....fluid levels, fluid temps, ambient air temp and pressure. Was the engine run recently, has it been sitting for a few days, has it been sitting for months? Is it fuel injected or does it have a carb? Is there fuel pressure, is there sufficient juice in the battery? Did the oil pressure rise when the engine started?...and so on...starting an accusim engine is just like that. It doesn't start the SAME way every single time as in a non accusim plane. Starting a real airplane engine is a major part of the flight and requires active participation from the pilot. Thats just one example of many as to why some of us really like the accusim concept.
There are a lot of simmers, like me, who are into that kind of detail, because it forces us to follow checklists, memorize procedure flows and such. This a great exercise in mental focus, keeping the 'flying mind' sharp. Obviously there are also simmers here who just want to press the ctrl e type starter and go....and that is totally fine, luckily there are addon makers who make planes to satisfy all of us.
A2A is the addon maker for those of us who want that kind of detail. This is what they do. PMDG makes highly detailed airliners...that is what they do, it is their 'mission'. Everyone knows this. Like PMDG, A2A makes highly detailed GA and Warbird addon aircraft. This is what they do, it is their 'mission'...to make the MS Tagline 'As Real As It Gets' stand up. Everyone knows A2A makes highly detailed study sim addons. Can you all accept that? Study sim addons are what they do, it is no secret. Accusim has been around for quite a few years now. They obviously do this very well with their large following. Can you just let A2A continue to do what they do best?
If you are not into highly detailed sims like that, that is perfectly fine, no one is saying you should be, or you are wrong for not wanting highly detailed flight sims, thats totally fine. No one is saying you are not 'cool' if you don't have the latest greatest super detailed PMDG airliner or A2A GA/Warbird. You participate in this great hobby of ours because you are looking to have fun, and that is wonderful, as it should be. For some of us too...the highly detailed checklist stuff is
fun for us as well.
Bomber12, Mike CYUL, Bazzar, Ant, Bill, Milton, the Milviz boys, Rob and Sean at RealAir, Razbam, Sibwings, Classics Hangar, Aerosoft and others all make absolutely wonderful beautiful looking addons that have the 'fun' mixed with varying levels of level of systems depth to fill your needs for having fun addons that don't need all checklist level stuff. You
already have a whole host of great developers giving you exactly what you want.
Please stop hounding A2A and PMDG because they are catering to the serious study sim crowd and not to 'you' especially when you already have so many great developers catering to you already. It makes you sound ungrateful to the guys who already do provide you what you like.
Cheers
TJ