This only makes sense if you ignore switching cost. For example, what if it becomes difficult to get your email out of Gmail (e.g. they drop IMAP support or throttle it).
If there is a switching cost, then you do have a vested interest in choosing companies with (a) sustainable business models and (b) business models that align their incentives with users' interests.
Advertising may satisfy (a) but IMO it does not satisfy (b). My reasoning is that if users pay directly for a service, the company is more likely to listen to their comments/needs/suggestions than if advertisers are paying the bills and the users are only there to look at ads (cynically).
I'm sure this doesn't play out this way all the time, but I do think it is generally true. There also may be some selection bias: people running companies who have user-supported services may have different values than people running ad-supported services. Again, definitely not true 100% of the time.
I think the best example of this is Twitter. Because they need ad revenue, they had to change the API and developer rules. Theoretically, this will not happen with App.net because users pay for the service so they don't have to worry about ads.
I think worrying about ad revenue introduces a set of priorities that may be at odds with what's best for users. There are certainly cases where ads work fine (e.g. Google search), but for something like email I prefer to not have to worry about this.
It is also worth noting that changing the revenue producers from advertisers to consumers doesn't automatically make a business model sustainable. In a vacuum, I have no reason to believe an RSS reader (webmail, instant messenger, or any random service) that I pay for is any more viable than a competitor that serves me advertisements.
Finally, the issue I take with your second point is that a company's motives are going to be dictated by their finances. That may even be a legal requirement if they are publicly owned. Both Fastmail and Google have a financial incentive to keep their user-base happy and growing. One because they are taking cash directly from users and one because they are using users to generate cash. I don't think that slight difference changes their motives as drastically as you might think.