As clanning has evolved, as the natural turn of events usually goes, 20 defence is now allowed by all pure clans currently. While the power creep has edged to 30 defence before, I believe that leaderships have learned from such and will not incorporate it again. 20 Defence, on the other hand, was a staple of pure clanning for many, many years without issue pre-eoc and will likely remain one for the remaining of pure clanning on oldschool.
This guide is to more-or-less convince you to either get 20 defence or to create a 20 def alt to use for F2P. It will be separated into two sections: clanning and personal.
Part 1 - Clanning
The way Runescape's accuracy works--without going too deep into damage formulas--is that If Attack Roll > Defence Roll, then proceed to damage roll, otherwise 'hit a 0'. Your attack roll is determined by your attack level and accuracy bonus (weapon used), while your defence roll is determined by your defence level and defence bonuses.
As seen above, there is a significant difference not only in the defence bonuses provided by a mithril armor set, but these bonuses also heavily outweigh the attack bonus of a rune scimitar. Of course, your defence is low and your opponents attack is high; however, this will still result in many more 0s than you might originally assume.
This 20 defence will not make you a good tank, but it will improve your tanking in F2P and make your food last longer 100% of the time.
In P2P, 20 defence is less of a deal-breaker. It does; however, give you access to significantly better magic bonus equipment in Xercian and Mystic robes.
Part 2 - Personal
For those of you without a maxed main, like myself, 20 defence allows for a mini-main type experience. Because of the slayer helm, initiate armor, mithril defender, adamant gloves, and other significant gear upgrades, it's much easier to slayer/mild PvM in your free time without bonding two accounts.
Items unlocked by 20 def include;
Adamant RFD Gloves
Studded Armor for N-Ranging
Slayer Helm/Slayer helm (i)