I have my rogue in the back line. Since I have not found a bow yet, and he runs out of projectiles pretty soon I gave him a spear to melee. So what weapon or skill category should I increase to make a character proficient with long reach weapons?

They are reach weapons which don't have a category of their own. So you can't improve them with skills but everyone can use them.

The only skill I could think of is Assassination at it improves melee weapons in general.

It annoys me a bit, but yeah, spears have no skill to them.
My original idea was to have a frontline Rogue, but since Assassination allows Rogues to strike from the back, and there's no range benefit to having a spearman in front (wth?), I ended up with two fighters in front, mage and rogue in back.

I found one and used it on fighter and will switch it when i managed to find a better weapon for them...

I havent checked this yet but: can't you use spear when you have it on first row and enemy is not in melee range? =1 space ahead?

Nope. That's what I had hoped for, so that normal weapons would reach 1, while spears would reach 2.

But nope. There's no benefits at all to having a spear in the first row.

I really would've preferred for Fighters to have a spear skill rather than unarmed, really.

In all honesty there's probably no spear skills because that spear is the only spear type in the game, comes in handy early on so your rogue/mage can do something if they have no spells or projectiles.

Sure they may not have fleshed it out more, but the game (at least early on) is better for having them at all than not having them :P

Later on rogue can use any weapon from behind (also bow etc) and mage will have enough energy and spells to deal with anything, rendering spears useless relatively quickly :)

Well, in all honesty, if there was a spear skill, there'd probably be more spear type weapons. It's a nonsensical argument whichever way you turn it. For whatever reason, they opted out of having spears as a skill and a viable weapon type.

I'm just having a hard time coming up with a viable rationale for that, other than that they really, really wanted each class to have the same number of skills and really, really wanted Fighters to have Unarmed. But I'm not sure I'd consider that good.I'm not sure I agree. If something is going to be included, I'd prefer it if it's included fully.It's not about Rogues and Mages specifically. It's about options. It's like saying that Fighters shouldn't have swords, because they could use Maces. Add to that that Fighters doesn't have any backrow options at all, which is just.. very odd to me, especially when I look back on older Dungeoncrawlers.