It's bad if done incorrectly, but there is a "correct" way to clutchless shift. With a clutchless shift, the synchros take the brunt of the force, but it can be minimized with proper technique to the point that theres really no big difference between it and a normal shift. Not like you're always perfectly rev matched when shifting with the clutch.
Try accelerating, get your right foot ready to shift up, then quickly roll off the throttle and click up a gear. When it's timed correctly, you'll feel it just slide into the next gear. It's all about tranny gear speeds and loading. What you want to have happen is to accelerate, pulling the collar faster. see http://auto.howstuffworks.com/transmission.htm
By accelerating the collar quickly and then rolling off the throttle, you're effectively unloading the transmission of the collar, get it? Initially, the engine layshaft and gears are pulling it along; when you accelerate and roll off, they pull on the collar initially, increasing rotational speed then when you roll off, the collar wants to continue spinning and is no longer being "pulled" as the gears want to slow down. You're unloading the transmission of the collar. Then you shift up and the slightly faster spinning collar slots into the next gear which is spinning faster than the first gear due to the gearing ratios.
Honestly, from the bikes I've seen, properly done upshifts don't cause damage; this is including teardowns, not just a "oh, the bike runs fine = no damage." It's fairly obvious when a clutchless upshift is done correctly vs when it's incorrect. Correctly, you will feel close to zero resistance from the transmission and you click up the shifter. If it grinds or doesn't budge or requires any force, you're doing it wrong.
But then again, everyone is entitled to their opinion and some just say they don't want to risk it. But I know people who've ridden for decades and never had problems stemming from clutchless shifting their tranny. And a racebike that only sees clutchless upshifts without problems on a teardown.
Editted: Just to clarify, I'm ONLY referring to clutchless UPshifts. Downshifts are another thing entirely. My advice, use the clutch and blip for a downshift.