I think both proof of burn and straight mining are perfect ways to distribute altcoins as far as fairness goes. Proof of burn is nicer for BTC holders, though, since it increases bitcoin scarcity.

So we can look at at least three aspects of the various methods of distributing a new altcoin:

1) Effectiveness for long-term success of the innovation

2) Fairness

3) Favorability to Bitcoin investors

My view is that the IPO method is worst in all three, straight mining is good for 2 but not so good for 1 and bad for 3, spin-offs are good for all three (though not 3 if you think Bitcoin distro is unfair), and proof of burn is decent for 1 and good for 2 and 3. As for sidechains, it depends on the implementation but it could be anywhere from bad for all three to good for all three.

Now 3 might seem like only a good thing for Bitcoin investors, but I think it does tie in with 1 because not detracting from Bitcoin's position tends to make it more popular with bitcoiners (note the near-total ban on altcoin submissions on /r/Bitcoin). Though it could also be argued that people who think Bitcoin is unfair or bad will be turned off by 3.

In summary, spin-offs seem like the natural way to do altcoins (in the case of spin-offs they're perhaps better called "alt-protocols for the Bitcoin ledger") now that Bitcoin exists.