There are situations where you can get major protection from team mates. If you’re always solo I’d prefer steadfast, for BGs or team play I’d go swift.
Looking for any guildies I used to play with:
Havoc Warhammer - Alair
LoC EQ2 - Mayi and Iskiab
Condemned and Tabula Rasa - Rift - Iskiab
Or anyone else I used to play games with in guilds I’ve forgotten
30% is a material amount of mitigation. I'd value the chance of it being up higher than permanent 10%, if the sets were my sole source of percentage mitigation.
But, Buffer is not a major/minor, so it can stack with other sources and become a material amount. For example, a Warden with Buffer and the the class armor buff would maintain 17-18% mitigation all the time, so depending on the class Buffer would be more valuable.