The roll to beat spell resistance is d20+caster level+spell penetration (if any). Even so, a 50 would be very hard to get past, unless, of course, the designers fudged things a bit for high-level enemy mages. Even if the spell gets through, you still have the saving throw (for spells that allow one).
In actual 3rd ed rules, the basic rollup is still with us. In this case, it's 4d6, total the three highest dice. The DMG has several alternate methods of stat creation, of which the point distribution is one.
I suspect the designers went with their modified version because overall, it's faster than rolling dice. I still prefer the dice.
Jack, I've bought that belt in HOF, too. Nice little item
Nightowl2