It would be a significant power-up to allow hit point rolls through level 20. For one thing, the flat hit point bonus for "unrolled" levels is a lot lower than what an average die roll would give you. Also, your CON bonus doesn't apply to "unrolled" levels but would apply if you rolled them.
For example, consider a barbarian with 18 constitution who is in "unrolled" territory. In the vanilla game he would receive a flat 3 hit points per level. But if you roll instead, he'd get an average of 6.5 from the roll (1d12) plus 4 hit points from CON for a total of 10.5.
Since barbarians stop rolling after 9th level, a 20th level barb would receive 7.5*11 = 82.5 extra hit points under an always-roll rule, raising his hit point total from 127.5 to 210. That's a gain of 65%.
You might be tempted to conclude, well, that's a big deal for a twinked warrior but maybe it wouldn't matter as much for wizards. So let's consider a mage with 16 constitution who is in "unrolled" territory. In the vanilla game she would receive a flat 1 hit point per level. But if you roll instead, she'd get an average of 2.5 from the roll (1d4) plus 2 hit points from CON for a total of 4.5.
Since wizards stop rolling after 10th level, a 20th level wizard would receive 3.5*10 = 35 extra hit points under an always-roll rule, raising her hit point total from 55 to 90. That's a gain of 64%, very much in line with the barbarian's 65%.
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For example, consider a barbarian with 18 constitution who is in "unrolled" territory. In the vanilla game he would receive a flat 3 hit points per level. But if you roll instead, he'd get an average of 6.5 from the roll (1d12) plus 4 hit points from CON for a total of 10.5.
Since barbarians stop rolling after 9th level, a 20th level barb would receive 7.5*11 = 82.5 extra hit points under an always-roll rule, raising his hit point total from 127.5 to 210. That's a gain of 65%.
You might be tempted to conclude, well, that's a big deal for a twinked warrior but maybe it wouldn't matter as much for wizards. So let's consider a mage with 16 constitution who is in "unrolled" territory. In the vanilla game she would receive a flat 1 hit point per level. But if you roll instead, she'd get an average of 2.5 from the roll (1d4) plus 2 hit points from CON for a total of 4.5.
Since wizards stop rolling after 10th level, a 20th level wizard would receive 3.5*10 = 35 extra hit points under an always-roll rule, raising her hit point total from 55 to 90. That's a gain of 64%, very much in line with the barbarian's 65%.