I'm not sure it does any more, but long threads can cause other problems.
The worst one I'm aware of is that people using IE 8 and Windows XP often cannot type text normally in longer threads, because it can take several seconds for the letters to appear on the screen.related discussion: Problems typing in longer text fields
That problem is fairly widespread and well documented, and is significant even when threads are only 100-150 posts long; but the LEO team has seemed unable to address it so far.
The 300-post limit, in contrast, isn't really a serious problem, since anyone can always start a continuation thread on the same topic. If you think there's interest in Merkwürdige Familiennamen 2, you could start it yourself. Just be sure to link to the previous one. (-: