The question I guess is though how much more secure is Spence really? Personally I think significantly so as I don't really rate Porro in a defensive sense. He's a hard worker and never gives up, he will chase back and harry, so attitude has never been a concern, he's just not particularly good either positionally or in a 1 on 1 against a speedy or tricky winger.
Now Spence I like the prospect of and on the right I think he should still be able to provide that 1 to 1 defending we so often see on the left but I dunno. Is he all I'm imaging him to be there? I'd love to see him get a start of two playing the RB, whilst he doesn't have the passing range of Porro I think he's a better runner with the ball and his crossing isn't poor from what we've seen so far.
I wonder if it's the lack of passing quality in the middle that keeps making Frank start Porro. Does he think we need that longer distance pass that Porro can also provide?