So in case they refuse to choose a side on a dispute to have it resolved, the coins will get stuck there and will be considered as a loss for everyone ?(for both the buyer and seller.. even for the escrow service provider, correct?)
yes. but it's very unlikely to happen with trusted agents.
+1: in case the seller offers his key to the service provider site, the site can run away with my money and then split it up with the seller as a part of the agreement, am I right ?
yep.
- Would you trust bitrated.com for not doing any of these with high volume trades ? (20-50 bitcoins)
no idea. depends on who you're dealing with.
just to make sure there's no confusion. it's not the site who's doing the escrowing but their users.