The only problem with using opennap is the multi-source downloading issue.
When searching for popular files it can sometimes be an issue because it means if you select an opennap result then you might be missing a file you can multi-source with.
For the rest of the time however opennap can be a benefit. There are issues with centralised network designs that use central servers, however there are also benefits. In terms of how well you can download stuff, centralised networks can not be beaten. Using it along side the decentralised WPN also eliminates many of the flaws with centralised networks as well, this means all of the benefits without the problems.
OpenNap is recommended for many users - there is only one problem that the opennap servers need to sort out, fake files... with central servers it is easy to filter them out and that's why they never used to even bother trying, however lately there have been reports of them flooding opennap servers with fakes, and those servers not filtering them out. I'm not sure how widespread this problem is but it certainly doesn't help, especially with patched users being fake free from the WPN so fakes from opennap would cause more of a problem there.
But as I said, overall it is generally a benefit to use OpenNap unless you are only ever going for really popular files in which case it won't hurt anyway, as long as you don't connect to any servers full of fakes of course.
as for the question of capacity... well, the more users there are, the more users there are that are likely to want to set up servers... it's a self-solving problem.
