It's not specifically that Rengar scales with Attack Speed - rather, he scales with on hit effects. Not only does his Q reset his auto attack timer as you say, the empowered version also grants attack speed. Both of those facts allow Rengar to make particularly good use of Madred's passive.
The issue with sitting on Machete and rushing your brutalizer is - what are you going to use those longswords for? You're Rengar - you're not gonna be ganking too often pre-6, and there aren't very many junglers that are going to realistically invade you. Unless something silly happens, your single goal in the early game is to hit level 6 as quickly as possible, with as much gold as possible. And with that goal in mind, Madred's offers significantly more than a longsword.
Also, remember - with Madred's, you're not just buying the stats. You're buying the time that you save by clearing faster. If it takes you an extra two seconds to clear every camp because you're opting to rush raw combat stats after your machete, you're effectively losing gold, and more importantly for Rengar, experience.
If I can get a level advantage over the enemy jungler (especially if I can hit 6 before them), the base stats that I get from leveling up are worth more than the gold I had to spend on Madred's passive. For that crucial level 6 gank, 325 Gold has essentially bought me 90 Health, 3 AD, 3% Attack Speed, 3.5 Armor and 1.25 MR over the enemy jungler, plus the extra damage/utility I get from the skill point. Which is pretty worth it in my opinion.