I'm sorry I have to disagree with you here bj. Yeah, while it's easy to remember those percentages, there a number of things here that aren't ideal.

1) If somoeone is going to learn something probably best to attempt to learn the correct and full way, imo. We all know poker is a game that requires studying, in order to get better. This falls in that category. If you can learn to do something the right way, why choose a less than ideal shortcut (which I believe yours is, will explain in next point).

2) The examples you provided are only for Straight, Flush, and Straight-Flush Draws. Which is 8, 9, and 15 outs. There will, however, be times when you have more or less outs, so just memorizing an "incomplete" chart is not going to help in all instances. (I of course say "incomplete", but it would be ideal to memorize the exact percentage for a given number of outs, however that would take loads of time, and we all agree approximates are good enough.) However, you must have approximations for all instances. And that is exactly what the "Rule of 2" and "Rule of 4" provides.

3) Obviously if you know you have a flush draw, you can deduct you have "at least" 9 outs to win the hand. Depending on where you are in the hand you pick which rule to use (think larger number = more cards to come. Therefore if your on the flop with 2 cards left to come you use "Rule of 4", and if you are on the turn with just the river left you use "Rule of 2".) After that it's just a simple multiplication of your outs to the rule number.

4) The harder instance if figuring out what percentage of the pot you represent in a percentage form, "imo". It's much easier to say there is 40 in the pot and you have to call 10, so the pot odds is 40:10, or 4:1. So if you can use the fairly simple equation to determine what odds you have off making your hand in ratio form you can easily compare it to the pot odds and decide your play.