I look for backs who have the ability to create opportunties for try scoring as a starting off point. Forwards are primarily to offer backs opportunties to score tries by doing the grunt work, defending the middle, making ground up the centre, tiring out the opposition. Sometimes you get talented forwards like Watmough, Stewart, Ola, who can create things, but that's a fortunate extra.
However, the backs are there to use the opportunities to score tries. The half and five eighth are there to create opportunities primarily for the centres to offer good opportunities for the wing men and full back. Defense is important but secondary in my mind, to getting the ball over the line. So its important to have backs who can create for themselves or their team mates a means to score.
DCE is excellent at creating scoring opportunities for himself including being a Terry Lamb like backer upper. Schuster creates opportunties for those around him and has the skill to deceive the opposition. Koula has sheer speed and evasivenss that creates openings needed for the full back and wing men. Garrick can create his own breaks, Saab just has sheer speed and Turbo just has it all.
The important ingredient is that the halves and centres can win games for you, if they have the skill. DCE, Schuster, Koula I believe can win you games. I suspect Weekes can also, but that's to be seen. Saw enough with that magic trial break.
That's what you need in the backs, Players who create situations that result in tries. Now I look at Parker and I see a hard working, reliable player, excellent defender, hard runner, usually wont let you down, almost a latter day Foran type. But I dont think he's going to win you games. He'll back up well, he'll hit hard in defense, but I haven't seen him create tries. Could be wrong. Haven't seen all the games, but a more creative centre would add another string to the attacking bow. He's not going to let you down mind you and if we had no one else of quality I would say he's the best option. Its just that I think we may have someone better