@YellowJay: This is a really late response so sorry if these points are not news to you anymore. :)
Shyvana needs about 50% ASPD in order to make use of her kit, most of which is on-hit damage. Her Q gets reduced cooldown on auto attacks. Her W applies on-hit damage with each auto (mostly useful for farming, not helpful in duels). Her E applies %-health damage on-hit. Her R cooldown can decrease from 100 seconds to 33 seconds if you're autoattacking camps between fights. About 50% ASPD is the sweet spot between lackluster Q/E damage (as well as R uptime) and diminishing returns.This means that unless you're getting Nashor's Tooth on AP Shyvana, you need early attack speed from elsewhere.
"Why not ASPD boots then and Cinderhulk for tankiness?" you might reasonably ask. Shyvana's snowball condition is to make it to end-game, but she can't do that with a heavy team that already got 1v4'd by the enemy jungler. READ: you are not an AFK-farming jungler, you are a power-farming jungler with strong jungle invades and 1v1 duels early-game. Don't get me wrong, you'll probably out-live the enemy jungler if you have Cinderhulk because it's tanky. You'll be hard to kill. These are all important, ESPECIALLY late-game. But survivability does not win you games--dominating your enemies wins you games. Because of this, your job is to capitalize on item spikes, i.e., early gold efficiency, and deny the enemy jungler a chance to get the enemy team ahead. Bloodrazor gives you this early spike: ASPD and a little bit of on-hit for high ultimate uptime, powerful duels, fast counter-jungling, and objective control. Cinderhulk+ASPD boots is almost 50% more expensive than Bloodrazor and you miss out on free %-health damage to high-health targets (e.g., the elemental drakes that Shyvana benefits so heavily from). It also locks you out of defensive boots, which can be detrimental against heavy CC teams or an AD that got fed. Although the combined ASPD stats are the same, you will likely be unable to get that gold AND score an advantage over the enemy jungler in time for the snowball.
NOTE: if you are gifted with niche talent beyond everyone else and you consistently (60% or more of the time) stomp the enemy team with Cinderhulk+ASPD boots or whatever else, then you can keep building those things. Across the board, though, it's highly recommended that you focus on early efficiency and get at least one early ASPD item, i.e., Bloodrazor.
There are other benefits of a Bloodrazor start as well. Firstly, you can build any combination of AD/AP and tank/glass if you start Bloodrazor because it's such a good staple item for Shyvana's kit. Moreover, if you go Cinderhulk then you miss out on the chance to glass-stomp the enemy team if you get ahead (because you don't need to be tanky if your team has 5 levels and 4 drakes on the enemies!). Also, you can sell your Bloodrazor late-game if you happen to pick up something like BoRK+Titanic and you want more survivability. Lastly, rushing the same staple item early game will teach you precisely how much your champion can deal. This heightened micro awareness of your strength will boost your decision-making ability on the champion, as you will more accurately be assess risk in your plays across the map. All of these doors open up to you with a Bloodrazor start.