A lot of this criticism is also assuming everyone is using their own computers. At a couple of places I've worked, we could store our files on a network drive but we got a fresh instance of Windows each time. Mailto links would bring up the Outlook Express setup wizard, which was pointless to go through considering it would simply be wiped again when you logged out.
On top of that, I know my mom, still fearful of Firefox, let alone Chrome, has a similar problem with mailto links and it is always quite excruciating watching her process for extracting just the email link itself without the mailto: attached to the front. Depending on who this is marketed to, I think some businesses could easily increase conversions by going this route.
Of course you have a fast computer. Of course you have a configured mail client (non web based). Of course you get your email client.
But what about the rest of the world?
People that have slow computers? Find programs (in general) intimidating and confusing? Are lucky enough to be able to browse the web?
What is easier for those people?
A modal that pops up immediately upon clicking asking what they would like to say or waiting for Outlook to load?
In contrast to more feature rich services like Wufoo or Jotform, it's simple, modern and it appears to work.
I say this is a viable service.
+1 for me.