I see their point, they may have a bunch of CSS hacks or other "tricks" to get the site to work with current mainstream browsers, but it gets broke in newer or other browsers such as Opera.

What is worse, telling the customer your site won't work, or just letting them cruise on in and fight and finaggle with it attempting to do something constructive?

I would rather be told, "Hey, you can't view this because of this...." rather than spend 5-10-15+ minutes navigating it and having the checkout fail or some other anomaly happen that angers me worse than them being up front and honest about it.

I think far too often the push of 100% availability to 90% of the target audience is just too unrealistic, and hey, even the most large and demanding sites have trouble and issues at maintenance at times. That's our imperfect world.