One drawback to magnet links is that you can not in advance see what files are there.
So if you only want to download some of them, you first have to wait for the magnet to download the torrent, then go back to it and pick the files you want.
A magnet link also makes it hard to check if the link you are looking at is a duplicate of what you have already.
With a torrent you can check the file size and compare to others.
If users only upload the magnet file, the TPB wouldn't even know how big the file is, unless their servers queried the DHT for the .torrent file and extracted the data.
So if you only want to download some of them, you first have to wait for the magnet to download the torrent, then go back to it and pick the files you want.
A magnet link also makes it hard to check if the link you are looking at is a duplicate of what you have already.
With a torrent you can check the file size and compare to others.