Width is the big one for boat entries. There are boats where it is not a problem, but for many boats it is a problem, and the diver has to fall in sideways. On a boat where the second tanks is stowed, sidemount kinda sucks, but BM doubles are no better because both cannot be racked. At least with SM, one tank is in the rack. It is harder climbing a ladder with SM tanks than with BM doubles, in some cases much much harder or impossible.
As a couple people have pointed out, if the gate is narrow, you just side step off the boat - it is really no big deal.
As for coming out, I can remember one exit in very short timed and nasty 8' seas coming up a ladder that was pitching though a 10' arc and coming entirely out of the water. That was in BM doubles and it was an interesting exercise in strength and timing to ride it up out of the water and the entire boat was lucky to recover everyone with no injuries. It's very true that I would not have wanted to come up the ladder with two side mounted tanks, but you have to take a very narrow view of diving to even think that this is necessary.
For example on a 4 bottle deep wreck dive in back mount, I'd normally take off the stage and deco bottle and either hand them up or clip them to a gear line before exiting so that all I had to haul out on my back was the back mounted tanks. The thing is that I'd do exactly the same in side mount, with the addition of removing one of the side mount tanks and clipping it of/handing it up as well. I'd have loved to have that option on the day with the short timed 8' seas as it would have made the exit a lot easier.
It's also true that narrow ladders can be an issue. But again you have the option of removing one tank and handing it up or clipping it off to be taken aboard later - just like a stage, a deco bottle, a scooter or a camera. It's not a problem it just requires a little flexibility in thinking.
It's unfortunate that some boats may be thinking of banning side mount divers. That's a reflection on poorly trained or inexperienced divers, not on the inapplicability of efficient side mount to dive boats.
And I agree with the posts above about the gearing up rapidly on the boat. I can configure my tanks and set up the regs in the back of my vehicle before I ever come aboard. As noted if racks are available one can stay in the rack and the other can be stood on the bench in front of it, then clipped and strapped to the other, although in most cases it just sits nicely under the bench along side my dive bag. When gearing up, I can delay almost until the pool opens, then hook up both tanks and be over the side in about a minute, two if I take extra time and chat a bit, even if I don't have enough space on the bench to sit.
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And that's some how different than a backmount diver going through a "backmount" restriction of the same magnitude, for a backmount diver, how exactly?
It's the same as the cave damage argument. To consider side mount a problem you have to ignore the same problem or challenge in back mount diving and that seems to be what Cam is doing consciously or otherwise.
It's true that we go places that are tighter than we used to go in back mount, but we still went as far we could get in back mount without doing damage. The same is true in side mount, the only difference is the ability to get farther in a much cleaner configuration.
We also have a great option in side mount that we don't have in back mount. It's like 4 wheel drive - if you stay in two wheel drive until you get stuck then engage the 4 while drive you'll really have to work hard or act really stupid to get yourself seriously stuck. Similarly, in back mount we don't go through a restriction if it requires us to take of a tank or swing a tank forward (although I will do that to go through a restriction where it is not required in order to just do it cleaner). That then allows us the ability to take a tank off or swing it forward to come out of the restriction, adding some insurance against getting stuck on the wrong side of a restriction.