I hope the O's do keep Markakis up despite his lack of professional experience. They need someone like him on the roster.
Looking at their outfielders: Patterson can't get on base, and though he has range, his defense is average at best. Matos can't hit worth a lick. Gibbons hits like an average RF (.270 avg, 25 homers) but is a bad fielder. Conine and Millar are old, old guys who can't move well, can't field well, and don't swing enough of a bat to play in the OF or DH. All in all, Patterson and Matos can man CF defensively, but have glaring shortcomings at the plate. combine the two, and you gave a much better player. Conine, Millar, and Gibbons are absolutely pedestrian and are all defensive liabilities.
I'm a big Markakis fan, and I think he's ready despite the lack of experience. He can develop further in the minors, but i think he's also the kinda player, who will continue to develop in the majors, without being rushed. He should start in LF. He has the batting eye and patience that Patterson lacks and the stick that Matos lacks.
In fact, I liken him a bit to Grady Sizemore. Same kinda skill set. More importantly, same kinda swagger and confidence, which will prevent the kinda slumps that plague rookies. Grady probably has a little more speed, but I think Markakis will hit for more power in the coming years. He only knocked out 15 last year in the minors, but he also had 40 some doubles, which is a sign of power on its way.
If you give the guy 450-500 major league at-bats this year, i think one can expect solid defense, a .280 avg, a .330+ obp (possibly even .350), 15-20 homers, 8-10 sb's, and more importantly, someone marketable who brings a little excitement & hope to Camden Yards.