If it's a like for like seat I say yes, otherwise no. When it comes to F if sitting together is that important than buy the cash upgrade rather than chancing sloppy seconds complimentary upgrades. As far as irregular operations yeah you often have to give up something to get where you're going faster. Different seat, loss of meal choice, less desirable routing, etc. Again, if sitting together is so important wait for a flight that has 2 seats together.