yes and no.
PvE
in the current meta you need to be a decent DPS/off-tank.meaning at team oriented raids you need to take at least 1 rank of peltasta in your build to get lvl 5 swashbuckling = AoE provoke ability to get monster to aggro you.
Maybe in future raids might need actual full tanks to finish them, but not in the current ones.
PvP
no pvp as of now, only duels, full tanks cant do damage dont count them to. in PvP arena they are useless and are often ignored and killed off later. in Guild battle they are surely the frontline fighters if they ever implement a castle siege, but no word on IMC as of yet.
HOWEVER
if going beyond peltast1, you know that irregardless of any game as full tanks and full supports arent going to deal significant damage and must know this. play full tank only if your always playing with friends/guildmates else your going to be a masochist if your gonna solo play a full tank.
ask @WatchGintama (the only person i know here that went full tank with high CON and with arena PvP videos)
ALTERNATES to PELTAST
None, as its the only class that can provoke and draw aggro to themselves. Centurion used to be the alternated but the class has been removed until further notice.
Rodelero might look like a tank but its more of a CC(crowd control) warrior that uses shields.
Paladin might look like a tank but its another support class.
classes with Peltast off-tank
Hoplite is the best option for a offtank( even full tank in theory) and works well with Peltast class. Finestra hoplite buff grants you with BLOCK rate and CRIT rate increase. if interested with hoplite see guide and ask question to [Guide] Hoplite - By Cathexis by @Cathexis
Peltast>Hoplite>Cataphract if you want to ride a mount.
Corsair2 if you want to wield daggers, you can use Swashbuckling with daggers.
sometimes Barbarians/Highlander (DPS classes) get peltast 1 for team oriented play, you can opt for daggers to not lose any DPS rather than shields that give no bonus offense.
Fencer is your alternate for a ‘evasion’ tank, but doesnt get rapier benefits if your equiped with a shield.