If ultimates simply scaled with the sum of your attributes then it would be similar for all characters of the same level and impacted more by weapon/spell power and penetration than the small total stat variances. Naturally this would in no way affect the normal skill usage which would still support pure builds over all others.
The only way to support hybrids for normal skill usage would be to have a passive available for a spellblade or a warriormage which base damage of a combination of attributes rather than a single attribute. Such a formula would need to be designed such that the hybrid cannot match the heights of a pure build, but can do better than they currently can.
This passive could go into a world tree with other suitable spellblade passives and maybe even actives.
I think trying to make hybrids based around dots would be quite difficult as not all classes have the same access to dots and hots. Basically it would be too big of a design change.
As for ultimates being based solely off level and not accounting for any build elements such as weapon power, spell power, penetration, critical and also ignoring all defences? I think it would make the current GOTO ultimates even more powerful and the others would be abandoned. Also it doesn't make much sense for a heavy tank build to have the same damage power as a full DPS build. Can you imagine the chaos that low cost ultimates could cause on a tanky build?
I think the best option is to balance it based on an average of weapon/spell power and penetration AND and average of stamina/magicka for offensive ultimates.