Well if you screwed up, it's not too late to turn into a tank. There are many pros and only one con.
Pros:
You won't deprive yourself with most of the game's contents. Tankers usually keep 70 Attack and Strength, and that's a level you can't achieve just by doing quests.
You will own in PvP and BH. Your opponents will struggle to hit you.
You can do GWD bosses for great cash, unlike some other pures who can't because of the lack of Defence level.
Cons:
You can't melee... Not while PKing anyway.
That's all I can think of now...By the way, get 71 Prayer for Ancient Curses (get that level after doing Temple at Sentisten and all the quests that give Prayer experience). Good luck.