[In response to mjohnson and Whale]
In my humble opinion, it depends entirely on the individual marine. If I was in charge of a team made up of five marines I didn't know, I think it'd be wrong to assume they all had more than basic medical training. Probability suggests one of them will, but I see no reason to expect your average marine to know any more about medicine than your average security officer. Basic life-saving skills and emergency procedures, etc.
Many marines might undergo extra training, which would enable them to fulfil a role as assistant medical officer, for example. But I wouldn't expect them all to be so proficient. Likewise, I'd expect a marine to be able to weld a door shut, or diagnose a basic problem with a warp drive, but I wouldn't automatically send a random marine to Engineering confident that he'd be a great deal of help to the engineering staff. Some will have that training, but not all. Same with all positions, really.
I see marines as specialist soldiers, most of whom have a decent level of skill in one other area (or perhaps they really did dedicate all their training to be an exceptional soldier, perhaps an assassin or bodyguard for a high-ranking figure. But these would be a bit more limiting to play in the sim I'd imagine). A bit like cross-class skills in an RPG. He might be a level 10 fighter, then take some levels in wizardry. He'll never be as powerful with magic as a pure wizard, but the pure wizard will be glad of his assistance in a fight for sure!
tl;dr version: What Whale said.

There's no reason marines shouldn't have secondary training in any duty post on a ship, but they should be written as if they're not as skilled at it as a character who's trained for that duty position. For example, if your marine has had some medical training, he's unlikely to be able to waltz into Sickbay and start diagnosing difficult patients that the ship's doctor was finding tricky (there are exceptions of course... maybe he's treated that particular problem before, or maybe his field-medicine techniques can be used to solve problems in ways a trained Starfleet Medical officer would never dream of using (the old biro in the windpipe type of thing).
Having said that, there's no reason at all a marine can't have additional skills, that come in handy in certain specialised situations. For example, back on the Aurora one of the marines had bomb disposal training. Needless to say, that became useful.

Specialist areas like that are great. But if said marine also happened to be an engineering genius in general, as well as a marine, that might be a bit Mary-Sue and render the actual engineers pretty pointless.
I think Kagran alluded to a good point there. It's difficult enough to come up with missions that give all of the 'traditional' duty posts something to do that falls entirely within their remit throughout. Sometimes, as - for example - a tactical officer you may be asked to do something that more naturally befits a security officer or engineer if there's nothing more tactical for you to work on at the time. Or the Helm officer might find himself helping out in medical (hi, Tom!) or engineering or even security. Even if they're not explicitly handed off to those departments, they'll often be given tasks to accomplish that are more befitting of that department.
This is even more obvious with Marines, as unlike all other duty posts, *any* mission that doesn't involve shooting stuff basically has nothing for your bog-standard marine to do. Therefore it's vital, in my opinion, that PC marines have specialised skills, so that they can remain relevant and don't end up running endless training sims in the holodecks.
You have a valid point, but it depends on the mission.
You can have as many combat training sims, and all be different.
It is all in the eye of the creator.
I for a fact have conducted many combat training sims on the holodeck, and all different.
I use them as part of my job in the security department to keep things fun, and some of the crew involved during a low point during travel from place to place.
It all depends on what the crew would like to do, and how it is conducted.
My marine was a counselor, a Klingon. So she would use combat training to get to know the crew under a stressful situation.
So there is nothing wrong with combat training sims, you just have to use the in moderation.
My current PC, is a Tactical/Security Officer.
Trust me, he is not perfect.
I didn't say there was anything necessarily 'wrong' with training simulations, and I totally agree that they're an important part of the sim for some characters. I was talking more about the situation where you haven't really got a lot to do with the main plot, some 'pure' marines may feel that training sims are all they CAN do in some plotlines, because there's no 'place' for them. Which would be unfortunate. Never said there was anything wrong with a good training sim now and again. Kagran was fond of them, I seem to remember. Almost got Landau killed in one, once. Only came down to fix the [...] holoprojectors!
Edited by Necessity, 17 January 2012 - 04:04 PM.