Do you think part of the reason why bruisers are going straight tank is gold cost
AP bruisers have similar itemization as other mages which is among the highest costing items. Similar issue with the AD ones also.
Compare.
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Vs
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Tanks have a similar general cost efficiency as ADC with their option of zeal items giving good power much cheaper. Tanks are beating their damage counterparts in being able to get their item spikes much earlier which not only come from the stats that the items give but the effects that people build them for also.