Many of the reasons I have for using SailfishOS have already been mentioned. The spying, the being patronised and being under almost total control of a few big greedy companies.
In addition, I have always liked the idea of using products as long as possible (even before everyone was talking about sustainability 
) and so when I couldn’t use my Nokia N8 much longer and MeeGo never had a chance, I was looking for an alternative (or a further development in this case 
) and came across SailfishOS, which didn’t require expensive hardware and promised to be open and community-based, both very good for long-term usage. What let me down in this respect was the hardware of my Jolla 1, not SailfishOS, so I decided to buy an XA2 Ultra (well, it’s an “Ultra” by accident, but anyway… 
) and that in combination with SFOS promises to be usable for quite some time.
I use a few Andoid apps, but I prefer native ones of course and I have the Android support inactive whenever possible. For messaging I use XMPP/Jabber, which leaves me with a choice of apps on any platform (not only my phone!) and servers, so no ‘vendor lock in’ at all! I’m really happy SFOS supports XMPP out of the box, but I think the support should be better (if you think so too, vote for Support encryption, file transfer and groups for XMPP/Jabber), so more people can be convinced to use it, instead of being sucked into proprietary solutions like WhatsApp.