The issue for many of us is we don't trust Google with our data sets as Google has publicly stated they scan data to identify targeted advertising activities as well as other data mining activities. This isn't to suggest that Microsoft is a corporate alter boy by any means, but at least Microsoft's current privacy standards don't reflect an ongoing data mining operation.
That, and just like with Microsoft and it's Office suite, there is a feeling Google's penchant to tie everyone in to their platform via their Android operating system, isn't necessarily good for a competitive environment.
And, some of us have just used Outlook so long (in my case over 15 years) that it just isn't worth it to change a PIM that you have spent so long customizing to get it "just right" for your needs.
But you trust microsoft with it, by running it through hotmail then out through another sync? Gotcha.. Microsoft tries to lock people into their platform all the time, so does apple, and so does google. I have personally experienced more issues with microsoft and email than google and email, and I won't use apple products so they are a non-discussion for me.
As with outlook, I've used it for years, with imap, pop etc in the past, now that I know exchange, I have found that outlook is meant for exchange, anything less and it can be easily replaced for free.
I use 4 different computers depending upon where I am going, and what I need, so I want to have access to all my email wherever I am, with google's 2 factor authentication, https to access it, knowing the data is safe (never had a issue with gmail since 2007). Hotmail has gone down, and stopped syncing because of to much data, exchange databases like to say there is corrupt data for no specific reason (after running database repairs for hours and having backups fail I know this), so for me, from personal experience not some story on the net, I don't trust hotmail/microsoft for important data storage.
If one is truly concerned about data security of your email, you would never let it onto your phone, ever.. That phone can become lost and your data with it. Use citrix or rdp and remote into the machine to access old email. And that should be kept on a encrypted drive using bitlocker or truecrypt.... But what happens if the machine gets knocked over, water spilled in it, stolen, burnt, flooded, whatever things can happen, one looses that data forever. The price of google possibly going through my email and looking at things for me is worth the piece of mind of knowing that if I need a email from 2001, that I can get to it.. Also.. you have heard of pgp for encryption? Use gmail for your sync, but encrypt the messages then it is still around, but totally secure..
Also about thunderbird calender being weak, or gmail's calender being weak. How so? One can share it out to other people, share appointments, do most of what outlook can do. What things can it not do? Unless one is running the business contact manager, it works for most people. I find that gmail, the contacts, and calender work for 90% of the people out there.
The moral of this story is, don't do this and that, then this and that, one will run into issues.. Kiss always prevails (keep it simple stupid). If you staying with outlook is super important and you want to keep your data secure, if you like android, pay for exchange hosting, it is how outlook is really meant to be used, or get a windows phone and plug it into your computer. But what happens if that computer goes bad? ......
I keep all my texts and call logs backed up on gmail also, sure the private data is there from my texts, I can delete what I want, but there have been more times then not that by me having it backed up to the cloud has saved my ass.
There is a price to be paid for privacy, that is ease of convience and use. But privacy in this world is dead anyways. Every time one does a bank transaction it is monitored, cameras on light poles, shopping habits, shoppers cards, email it's self, data going across networks, cell phone calls being tracked, etc. I just assume that at anytime a government agency can be monitoring my cell phone calls, debit card transactions, or whatever.