/opt is not a linux standard. (well.. actually nothing really is a "linux" standard ).

When you install any Linux system it actually will let you choose what partitions to create.. using something like /opt is only beneficial if you actually separate /opt from your root partition.

The idea behind /opt is that if you fill that up 100%, it won't affect your root partition (since it's a separate partition) and therefore your device will still boot, you just won't be able to install new packages. However, if you setup your system to just have a / partition.. then it doesn't matter if you install apps to /usr, /etc, /opt, /sfw, /somewhere/noone/cares/about ... they are all on the / partition and will still fill up the drive.

The vast majority of software available for maemo I believe is a "port" of a desktop Linux application where /opt is more or less not used. Some of the more common desktop linux layouts for hard drives are any combination of these 6:
/boot
/home
/var
/usr
/
swap

So on Desktop Linux.. installing to /usr could (if the user set it up that way) be just as efficient or system-friendly as installing to a /opt partition. The name is irrelevant, it's just whether it is a separate partition from the root (/). I would venture that the *most* common setup is simply a root (/) partition, with swap on a desktop. That's subjective though based on my readings of how-to's and tutorials.. and is the one I personally use because every time I try and break it up to one of the ones above I almost always wind up with too little space in one area, too much space in another.

Doing that, however, means I have to keep an eye on my disk usage as I could fill up my root with both my Home, and my software. So really.. this "/opt" thing is more for mobile or low space devices. In theory, it's great.. in practice - they probably caused more problems branching /opt from / than if they just left the whole 32GB available to /.

As far as your questions relating to identify non-optified apps see here and here.