What are some reason Xerath isn't used as support more often, he seems to fill the same role as Brand support but has more cc, a stun that doesn't require another ability to be hit first (although the cooldown is a bit longer), and longer range poke?
There is a very simple reason: Xerath has very high scalings and mediocre base damages. For Brand and Velkoz, add in their passive that doesn't scale with AP at all. Xeraths Q has 240 + 75 % AP damage, and similar abilities have 260 + 65 % AP.
As other ppl say, his CC is also lackluster for a supp, a little weaker than Brand.
Because there are plenty of options that are better than him. He only has one single target stun that is a skill shot. He needs item to be really effective and that's not easy to obtain when you're a support.
0.5 second stun sure is going to save your ADC!
Might as well just use Morgana or Lux support.
Xerath is a bit item dependent, but he makes up for it by not needing a mana item as much as his other mage counterparts. he has longer range poke, and his burst combo can hurt a lot, although others can do this in an area as well.
I played Xerath as a support a few times, and I enjoy it because I can rush a Rylai's an not have mana problems, while still helping my team with the slow from my low cooldown Q. I'd pick him against Lucian, Kog'Maw, or Vayne, as he can stop them in their tracks really quickly. He doesn't have any other way to follow up though, so save the stun for when you need to not die.
It could be a thing. Let's make it a thing.
Unless your going to 100% win lane against the enemy duo lane It just won't happen as healing support just out weigh zoning supports if they can't burst them down or roam effectively. He also is very immobile meaning him roaming and placing wards puts himself in a very dangerous position.
because 
are way better AP supports.
Xerath doesn't have enough utility or damage to peel for a his carry in teamfights. His mana costs are also pretty high for a support.
I would say that xerath poke pushes the lane and usually that isn't favorable for an adc
Lux does everything Xerath support does, but with a shield.
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What are some reason Xerath isn't used as support more often, he seems to fill the same role as Brand support but has more cc, a stun that doesn't require another ability to be hit first (although the cooldown is a bit longer), and longer range poke?
he was until real supports came back into play and then he couldn't match up
He needs farm, and Karma does his job better (in the early game)
Xerath is more mana hungry than Brand, has a worse autoattack animation (this actually matters in bot lane) and doesn't have DOT to delay recalls.
Too much collateral damage, he's very likely to kill minions which would reduce his gold income, and annoy his marksman.
Xerath + Varus bot lane is hilarious btw
Because Xerath doesn't do any damage without a full AP Build, unlike Brand and Zyra that build Rylai's and Liandrys and outdamage every one on the team.
Also, despite his passive he has a lot of mana problems.
I do play xerath supp. You make up for the low base damage by taking cdr runes and using the passive to sustain through constant casting. take a dark seal and play safe to make up for the gold deficit. The support quest completion makes frost fang extremely strong on xerath since it gives him a much needed escape tool.
Because his ult is more useful for a teamfight
i have been maining xerath support lately and im raping with him...like 80 percent win and a KD over 3.4 lmao...jihn xerath is stupid bot lane
The big reason is he doesn't have a whole lot to offer outside of damage. Vel'Koz and Annie offer more reliable poke and peeling abilities with similar levels of damage, at the cost of a little range.
He used to be more of a support, but he doesn't offer the dmg a brand provides or the consistency. He can sup, just not as good at it as brand. Plus he is super susceptible to kill lanes.
because an adc can easily kite him and stick to him, he has no means of escape, his Q slows him down further. the only saving grace would be his E stun which isn't as reliable as you would think.
overall dont put him anywhere other than mid lane, he needs the protection.
