Google makes using Thunderbird for your Gmail possible. If they had a check-off box that said "Show No Ads", there'd be nothing wrong with checking it. However, that's a wholly different thing than going to lengths to disable the very unobtrusive ads shown in exchange for using the web client.
Not all ads are paid-per-click, some are paid-per-view. And even for those that are paid-per-click you can often get into better ad programs if you have more ad views.