I could see uptime not being of extreme importance in the cases of sites that are not commercially oriented. I also understand that far more is taken into account when determining overall website effectiveness, such as quality and usability of the site, effective calls to action, useful information, and accessibility via referring sites.

That said, none of those other effectiveness characteristics matter if the site can’t be accessed. Additionally, for those using a website for commercial endeavors (either for direct sales or driving traffic to another location to conduct business), any time that a site is not up is a time it is not available to customers. Although no one can guarantee that a site would be producing sales during downtime, there is at least a possibility that sales are being lost due to downtime.

To answer your specific question, however: I would say that uptime itself is not a good measure of website effectiveness unless the other aspects of that site are functioning optimally (or even just adequately).

http://gigaom.com/2009/02/18/when-it-comes-to-social-networks-uptime-doesnt-matter/

I would say that it depends on what you are looking for. If all a person is looking for is a host that will be up most of the time, and that person isn’t relying on his or her site for income purposes, then it’s probably fine.

But if you are like me, and you’re using a website as a way to drive business then uptime is critical. The amount of money a website has the potential to generate in a single hour far surpasses the 5% refund that an A2 customer might receive. So in my case, the 99.9% Uptime Guarantee is a terrible contract; it gives me nothing of value and gives A2 every possible way out of honoring the guarantee.

Another good way to think about these things is to compare them to situations you might experience in other settings. When you get your hair cut, if the stylist leaves a chunk uncut, just draping down your shoulder, you don’t expect to pay for it at all, much less 95% of the price. And you don’t have to let another member of the staff agree that it’s not right prior to agreeing to the measly discount. If I had the same “guarantee” from all my service providers (lawyers, doctors, hair stylists, teachers, clergy–can you imagine only being partially absolved after tithing!?), I wouldn’t be nearly as excited to hire people to perform those services for me.

So I would say it is a mostly-bad contract, and, personally, I never would have signed off on it unless I knew that A2 was specifically trying to ensure that their guarantee would never actually be utilized by anyone.