Mage classes like transmuter, conjurer, invoker are already very powerful as-is. The downside tends to be 500 base hp at hero, so you're always counting on having top-tier gear to take you to a more-bearable 1000 hp at hero. So you'd have 700 hp regear, 1000 hp top gear. With Soul, it becomes 900 hp regear (!!), 1200 hp top gear.

Since hp is amplified by damage reduction, it works on mages even better than it does on low hp warriors!

I've never played a necromancer but I hear that they're very powerful too, so they probably benefit lots from +200 hp.