Since getting back into the hobby last year, I've been debating whether or not I wanted to go with proto:87 standards. I'm not planning on modeling steam, although it will take some work to convert the diesel locomotives.
There are several potential downsides:
No commercial turnouts, etc.;
Nobody can run their equipment on my layout (or vice versa);
Nobody (that I'm aware of) has built a large layout with proto:87 standards.
The fact that nobody has built a large layout to proto:87 standards is actually a kind of incentive to me, to prove it can be done.
I'm not sure if I'm really concerned about operating equipment on multiple layouts. Except for locomotives, all I'd have to do is swap wheelsets anyway. In addition, I'm using Sergent couplers, and I don't know anybody else who is.
The only real concern then is using commecrial turnouts. I will be "handlaying" my track and turnouts with Central Valley products. But I'm planning on using commercial turnouts in the hidden staging areas. If my handlaid turnouts work as well as they should, then this isn't an issue. So we'll see what I end up with when I start laying track.
If I don't go with proto:87 standards, then I will use HO:Fine standards, with .088" wheelsets, etc.