Its all about balance folks. Warriors and Thieves have to max several skills in order to be effective at what they do and Mages have to as well. As a mage, you can't just expect to max destruction magic by itself and dominate.
Warrior classes have to max 1h or 2h weapons plus an armor skill plus smithing and archery to be effective.
Rogue classes have to max sneak and 1h/archery plus light armor skill and possibly speech or alchemy to be effective.
Likewise, Mages are requires to max out Destruction, Conjuration, Illusion and either Alteration or Restoration.
Fair is fair, you can't expect to jump into the game and max a single skill and kick all available ass. You need to use all the skills you have available if you want to be successful.
I totally agree, but tell me how that logic extends to Magicka regeneration being significantly reduced in combat?
Why would that happen?
For thieves and warriors, there is no equivalent limiting factor which governs how effective you can be. Stamina doesn't even go down when you attack normally, and it's regeneration rate is the same in combat as it is out of combat.
At higher levels there should be more focus on using different kinds of magic to get the desired result - fine, I completely agree with this. But given that there are limited perks, some of the schools of magic are VERY expensive.
This means that if (in my case) you've specialised in Alteration, Destruction, Enchanting, Restoration and to a lesser extent one handed (only one perks thus far, I can't really justify more on reflection), it's not really practical to apply any perks to Conjuration or Illusion, which are certainly needed in conjunction with your specialised skills.
When you cast powerful versions of those spells without perks, you have to wait for ages for the bar to go back up because regeneration is much slower in combat, making those spells considerably less effective.
I totally appreciate that the game is about specialisation, but I just can't figure why they've done this. Why would regeneration be quicker out of combat than it would in it. Or if Beth have chosen to do that, why not do the same with Stamina?