But necromancers gain the same feature... just much later. Is this better?
It's not just a matter of taking it later. You get two schools (the second at level 4), but you can only master one, so you have to choose to master necromacy (and therefore lock in your paragon path as well) to get the benefit, while a pyromancer could just set it and forget it. Also, the necromancer gets something else at level 1. For a pyromancer, if you go up against something without fire resist, you basically have no benefit [other than the small damage boost]. The other schools don't rely on the type of enemy you are facing to benefit from your apprentice mage class feature. So, getting it later means you get something else sooner. Depending on what that is, it may be better. [Also, you can pick a different mastery option, so you may have a choice to pick a better option if the necrotic resistance hasn't been a problem and you don't forsee it being one in the future].
And yeah, I like pyromancers.
What is bad about able to do damage with thematic spells?
Pyromancers are tied to fire spells. All fire spells deal fire damage, but not all necromancer spells deal necrotic damage. Also, most fire spells don't do much other than damage (or conditional damage, or save ends damage, etc). No stunning, dazing, weakening, knocking prone, penalties to defenses/attacks, etc, etc, etc. So, if the only thing fire spells do is damage, making sure they do their damage is a good thing. Necromancers as a school is likely not as focused on damage as the primary focus. We don't even know what they do get as their apprentice mage feature yet, probably something that makes their spell
effects better instead of just improving damage. Pyromancers are secondary strikers, necromancers aren't.
Is it fun for the group when the controller nearly sucks?
For the controller?
For the DM who laughs at the character and his own wisdom to use fire-resistant monsters?
It makes the fire theme able to compare to a radiant theme.
BTW, Assassins are able to ignore poison resistance and immunity for just 1 feat!
Pyromancers definitely needed their ignore resistance (although the flat resist vs. a scaling remove x ammount would have maybe been better, but just flat ignoring it is probably the better option just for the purposes of simplicity).
As for poison resistance ... it's rarer than fire/necrotic, and I'm not sure, but it seemed like poison immunity was more common than poison resistance (as lots of undead and constructs were just flat out immune to poison).