i wanted to know for what the points over 50 are useful for a caster?

increase dammage? increase mezz time? increase chance to hit with dammage? increase chances to mezz? increase debuff?

increase damage?
--> Yes, but very little

increase mezz time?
--> no

increase chance to hit with dammage?
--> no

increase chances to mezz?
--> no

increase debuff?
--> I don't know, debuffs seem to work different here anyway.

Wasn't it that the higher the skill in a certain spec the less the variation of
it's dmg spells? Think thats the only point. For all other spells like debuffs...
They have got a fix value. The only possibility to lower the resist rates of
certain spells is the RA "Mastery of Focus" that raises the spelllevel of all
spells only for resist-calculations until the theoretical spelllevel reaches lvl50.

The only possibility for casters to increase their dmg is by RealmAbilities.
There are three ways...

Mastery of Magery (passive) - increases dmg by a certain amount up to +15%
WildPower (passive) - increases chance of crits with spells up to 39%
Augmented DEX (passive) - raises Dexterity which increases you castspeed

The best option is a mix of augmented dex + wild power at first, giving you
the possibility to crit and augmented dex increasing your damage-per-second
extremely. On live the maximum useful DEX is aroun 400 including TOA
castspeed boni.
If you believe NO WAI, Mastery of Magery is much better I recommend you
to make a caster on that RR10 server and take aug dex. Most players there
are casters and it is absolutely insane to cast faster than your castanimation ^^

Greets
Sethor

Augmented Acuity works as well with increasing your casting damage, as bonus you'll have more mana, too

@Sethor: if u have 50(or 51, not sure) u have no more variaton with your damage spells. more then 50 increase damage, but not very much, as gerbald says.