No its not. Illusion or Conjuration don't need the other spell schools. Its been tried and tested on Master. And if they do, If they choose destro over bows they are gimping themselves on Master.
Also take into account archery/melee + the 4 other schools can result in a superior character than destro + the 4 other spellschools; as you'll have more magicka to cast them while putting out more dps.
Its just an isolated issue with Destro scaling and dpm (damage per magicka) resulting in a very unfun character and forcing you to to xploit crafting.
The strongest build that does not rely on exploits or Shadow Warrior (to abuse terrible AI) is simply a straight up Archer build that uses Conjuration up to Twin Souls or a "Warrior" with the same investment into the Conjuration tree. There is absolutely ZERO reason to play as a "Pure Mage" other than RP reasons. Illusion can also be abused for both sneak attacks as well as just manipulation, but it's not needed and is actually slower than physical + Conjuration.
I know, the game isn't about "big numbers!", but that's not the point. The point is that it's infinitely slower and annoying to actually play a Mage when they should have as smooth of a time as anyone else with their damage school, but they don't.
I honestly don't understand why people say destruction is bad, those runes and saved my life more than once. Level 41 now on my mage and have yet to have any of the "underpowered" problems that many people have been talking about. Just enchanted some apparel, get some buffs and I'm good to go.
People are probably just mad that they can't one shot everything, or just don't know how to play mages.
I puke a little everytime I see this same, inane and condescending remark. It's just so laughably off base to assume that because you are not cognizant of the problem, it does not exist and that it must just be the mental defect of others that is giving them the "illusion" of a problem. The game is not hard, and playing a caster does not make you a special little snowflake.
As I have read from others, things go haywire around/after level 50.
This number seems to rise and fall depending on who posts it. The difficulty also plays a huge factor, because it's well before 30 on Expert.