THX insures a minimum performance level and for those not well versed in AV equipment, may be a safe way to get a fairly decent quality product without doing any extensive research and that could be important.
That being said, there are many high quality products that do NOT carry the THX logo and will meet or exceed the performance of THX certified devices but prefer not to pay for THX licensing.
THX is no longer affiliated with Lucasarts or any of that type stuff, and as such I think they aren't held to as high of standards as they once were. I wouldn't put too much stock in whether or not someone has submitted thier product for external review by the THX board.
Some recievers have a THX mode too, it's just another surround simulation technique like Pro Logic.
pro logic not just a simulation technique, it was the standard for a while. thx has never been the standard like dolby digital is now. to answer question thx is not some sort of standard, it is certain specifications that sound (amplification or speakers) must adhere to to be able to receive such label. herein, a cost is associated to the manufacturer to label their product thx certified to some level or another. my opionion is that no.. its not important to have thx certification as that would just add to the bottom line $$ of the equipment i were purchasing. when buying decent equipment, i just listen as should you and any other buyer. do not worry about specs as long as the feature you seek reside in the product you desire.