Switching up the format a bit, instead of individually saying yes or no to particular players, going to compare a couple people that are trendy waiver wire pick-ups to see who is the better option.
Roark may only be 19 games (10 starts) into his career but so far he's taken well to pitching in the major leagues. While he did have one tough start against Atlanta, a lot of people seem to be in buy high mode as he's coming off a complete game shutout against the Padres.
The Nationals are already winners as all they had to give up to get him and another minor-league player was Cristian Guzman. Even if his stock is inflated by having the good fortune to be coming off a start against whatever the Padres call their starting line up, his strong performance appears to have some legitimacy as his FIP of 3.43 indicates he would be a decent option for your team.
His strong start doesn't appear to be a byproduct of being lucky as his .297 Opponent BABIP indicates that not much (if any) regression should occur and all of his starts (with the exception of the aforementioned one against the Padres) have been against teams that are currently .500 or better. If your short on pitching, Roark appears to be a legitimate option.
Markakis got off to a slow start as his OPS was below .600 as recently as April 16th but getting at least a base hit in 12 of your last 13 games and homers in both games of a double header has a way of getting fantasy owners attention.
Since he's been in the league for nine years we have more data to see what type of production your likely to get out of Markakis. You'll likely get slightly above average offensive production (114 career OPS+), with a respectable batting average (.290-.300 range) and moderate power (around 15 homers a year).
Who Wins the Waiver Wire Cage Match?
On one hand, you have Roark who is relatively new to major league hitters. While there is a risk that hitters will successfully adjust and figure him out, his FIP & BABIP numbers indicate that his early success is not entirely fluky and should be a solid contributor in the Nationals starting rotation.
Markakis on the other hand, while he is a known entity and a fairly productive player, he isn't a major superstar and there are plenty of outfielders available who can put up modest power numbers and offer the advantages that he has.
I'm siding with Tanner Roark as at this point, its harder to find solid starting pitching then an outfielder with decent average and moderate dosages of power.
Winner: Tanner Roark