Bad HDMI/DVI cables does definitely impact the image as a whole apart from sparklies. I had one cable which caused sparklies and changing that one it actually felt like the image was more full and relaxed.. I don't know how else to explain it.
Quality boosting can't hurt right?

Well if it's unnecessary it can hurt the wallet! :-)
I own two Oppo HM31s, and I just read that it has a signal equalizer, and they similarly claim, "The signal equalizer is a key component to support long HDMI cables. It removes distortion from the input HDMI signal after it travels over long cables and restores the signal quality/strength."
http://www.oppodigital.com/hm31/hm31_support.aspAs with Meridian's claim about the 121, I don't think that this is accurate. How these devices know how to restore data that has been lost or distorted before it reaches it? That doesn't make sense to me.
If I understand HDMI correctly, the problems we see are a result of the signal being too weak, or there being an actual fault in the cable. The malfunctions we see are a result of the TV trying to make due with the poor signal it is receiving - not that there is interference, or actual distortion in the signal. All these signal boosters/equalizers are really doing is restoring the signal to a level that the receiving device can properly decode. I would imagine that a faulty cable cannot be fixed with a booster.
The Oppo runs for $99 compared to $795 for Meridian's 421. The Meridian has some auto switching for a Meridian DVD player, but it also has the buffering and de-jittering. Does no one know how, "proprietary active termination,
automatic cable equalisation, buffering and de-jittering technology visibly improve image quality, reduce noise and stabilise the video - even on short cable runs,"?