I find this rule highly annoying and I don't see the point of it either.
Oftentimes you want to react to the whole lot of a previous posting but with 3 sentences the essence gets lost and there's no point in replying at all anymore.
OR you have to create 3 line quotes within your reply again and again which is awfully time consuming and annoying.
If it has to do with server space... get rid of old topics or get more server space.
Many people have coughed up money, so much that we were told it was enough for years. Then the least you can do is make sure we can use the forum normally.
And implement the policy -which can be automatically done even- that older & inactive topics get removed.