Sione is definitely worth persevering with, he's still very young and yes he has some weaknesses in his game but he was also a big part of a lot of the good stuff that we saw from our team this year. If nothing else he is a very wholehearted player with a great attitude which is something that you can't really put a set value on like you can with other stats.
Obviously if we're still having the same conversation about his defence and tackling technique in a couple of years time then I will be a lot more concerned, but at this stage I'm happy to let him develop these areas of his game.
For the record I would like to see him stay in the second row, it suits the hard lines he runs and he's probably not quick enough to play centre.