I'm also not sure where you get that statistic of 90% from and the evidence it applies to viewing adverts on a photo sharing site
There are two main ways to consider this -
The adverts you see yourself if you don't purchase an account.
These are served by yahoo, a major international organisation who are rather unlikely to advertise anything inappropriate for fear of scandal or further revenue loss from people ditching the platform. I also think the cases of genuinely upsetting, inappropriate and offensive adverts are really rather low, ESPECIALLY for an organisation that makes a good chunk of their revenue from advertising.
As we've said, their ad targetting also mostly uses your browsing habits too - I tried logging out of flickr and ended up seeing a load of amazon & camera adverts. Irritating, but as you say we naturally tend to ignore them. Furthermore, if you DO get ads yourself that you do not wish to see you can adjust the adchoice settings to block them. So it's either targetted & ignorable or changeable & ignorable, for your personal experience.
Using flickr to host your images and others seeing "bad" adverts and associating them with your images
Almost the same logic applies. I think you're focusing on a exceedingly unlikely worst case scenario... What are the odds of someone browsing your flickr photostream, getting served an advert that is so offensive they consciously or unconsciously become so offended they become so irrational they make a link on some level between you, the independent user of Flickr, and the advert you have no control over, and then do something about it other than move to a different picture?
I really, really don't think that the advertising yahoo uses will have any measurable effect on association - positive or negative - between your work, the advert and the viewer. And I still maintain that if someone does make this extremely tenuous connection on some level AND makes a rash decision about you, chances are they are a complete and total buffoon and you won't care much about their opinion anyway. The vast majority of people using flickr who are likely to find you are likely to be photographers with a similar mindset to you anyway.
Even factoring out the adverts disappear if you pay, there are other benefits - two terabytes of always-online, accessible anywhere, enterprise-level backed up photo storage, monitored by a number of photographic agencies worldwide that presents your work in an extremely attractive fashion, allows you to join any number of other interest groups to share images according to personal taste, can lead to friendships both online and in the real world, hosts the illustrious Croydon Camera Club group pool and - most importantly - let's you see my work in all it's glory. What's not to like
I think it's an absolute bargain.