Throwers are a little unusual on "bash" teams like Orcs and they are one of the things that make the team unique. Actually having players start with Sure Hands and Pass, along with passing skill access, gives Orcs options for developing a passing game that just aren't really viable with most other teams. For example Dwarves have their runners - but only having four players in the squad with AG3 limits the passing options somewhat.
Throwers give Orcs the potential to develop a genuine long passing game. Accurate & Strong Arm throwers (requiring a double) are throwing short passes on a 2+. With several AG3 outlets (blitzers normally, but line orcs and goblins too) Orcs have the option to play the game both ways.
That said Orcs are an excellent bash team - but a mediocre passing team. They are just too slow and unreliable (AG3 and no catch skill, combined with AG3 passing). If you want to build a genuine passing team play elves, skaven or humans. What the passing game gives Orcs is a decent back up plan should the running play stall or should a faster drive be needed. You don't have to do this - taking goblins with catch or a couple of +AGs can make passing a real threat. However other teams will do it better and its not really playing to your strengths (so use it to exploit opponent's weaknesses!).
Throwers are actually slightly overcosted. Compared to a human thrower they have -MA and cost the same and have the same skill access. MA5 and Av8 are key concerns with the player type - they are comparatively slow and soft. However Sure Hands, Pass and passing skill access are worth something to a team that without them would be fairly one dimensional.
That is all very important because a lot of it affects how to develop throwers and whether to take them or not. There are some coaches who prefer to emphasise the running game and as such feel that throwers Av8 makes them comparatively weak compared to the cheaper line orc. This is particularly true in tournament play where the short format generally prevents the development of a better passing game. 20k saved by taking a line orc over a thrower makes a big difference. My preference is to run a thrower anyway because Sure Hands is a good skill to have. The reroll is useful and often you'll end up facing War Dancers with strip ball (which sure hands negates) so throwers as runners can compliment the running game well.
There is an argument to say give a Blitzer Sure Hands and use him instead. Its slightly more costly (10k premium for the Blitzer with sure hands vs the thrower with block) but you do get +MA and +Av which make it look like a bargain. The counter case is that this ties up one of your better players as a ball carrier reducing hitting power.
In medium-long term league play I really can't think of a good argument not to take throwers. The premium is relatively small and the options it gives are well worth it. The question then becomes one of development.
This is the classic build - Block first then skills to help with carrying the ball. It is pretty much the automatic first choice. After that Kick Off Return is good for improving coverage of the back field and putting the ball carrier 3 squares further forward on kick off. Fend helps if you do get hit and reduces the amount you'll need to dodge.
Doubles dodge and sure feet are the preferred options as one helps you survive hits and escape from trouble, whilst the other helps compensate for the low speed.
Nerves of Steel can also be handy for getting out of a tight spot, and Leader can help if you are low on rerolls. However I generally don't bother with leader on Orc teams as you are generally cash rich and a useful skill on the thrower can add more value to the team.
Accurate is the obvious first choice if you want to develop your thrower into a genuine passer of the ball. 2+ quick passes should mean you are throwing a completion every drive just to speed up development. On doubles Strong Arm will help develop that long range passing game. After those two the options aren't so clear. Kick Off Return is great - but you generally only want it on one of the throwers. If you are going to build one passer then my preference is to give it to them since you'll normally want them picking the ball up and passing it to a runner. Safe Throw helps with long range passing because it means you need to worry less about interceptions.
A defensive thrower is a build type that is a little bit unusual. Orcs don't really need two throwers on offence - particularly if most of the carrying is done by Blitzers. So that gives the option to build a thrower specifically for defensive duties. The key skill here is Kick because you don't really want to give it to anyone else. I'd take Block first because its so generally useful though. Then you'll follow up with skills that aid in retrieving the ball - Nerves of Steel being a classic option so you can try and pick up in TZ (with Sure Hands!) and then if you manage to you can dispose of the ball safely.
I'm putting this build down out of completeness, but its for a play style I haven't really tried myself. The idea behind the Option thrower is rather than to try and protect the ball carrier you use the ball carrier to suck in the blitz to open holes elsewhere to exploit. The key skill here is Dump Off combined with Nerves of Steel and Accurate to make the play more reliable. The thrower is exposed to the blitz, and then dumps off to a nearby receiver who can't be effectively marked. Personally I don't think Orcs are a good natural fit for this style of play - not having very reliable receivers - so I'd rather build to protect the ball carrier and pound the opposition.
I generally like starting teams with one thrower and building them as a dedicated passer. The 2nd thrower I'll normally build either as a Runner or Defensive thrower (or a bit of both). Then the passer gets completions to the runner so they both keep developing and even up.
All in all I think Orcs are very lucky to have throwers. They provide options that would otherwise be unavailable to a classic bash team and mean that when needed they can up the pace to score quickly. This is one of the key advantages they have other other bash teams - they have a Plan B that can work well.