I have mixed opinion on alts - i fear the exception becoming the rule.
If we're short of geared mains for a particular role i see no problem in bringing an alt. If the alt can do comparable dps to the main, i see no problem bringing the alt. Both subject to the usual main > alt loot priority.
What i do have problems with is people only signing up if they can bring an alt, or swapping out 5 different people for each boss. What starts out as an idea to improve signups i can see very shortly causing aggro about increasing the time taken for runs, or even more trouble trying to get a balanced run.
Certainly for stuff like fester/rot blood, we could do a raffle/dkp auction on the forum if theres many that want it for alts - set a 100dkp min bid or something, require the alt to do at least 7k dps raid buffed, and see how it goes.
And to be honest, we still have problems with fights like putri/BQL/sind, so unless your alt outgears/dpses your main there should be no place for them there.
In an ideal world there'd be no one who stops signing as soon as they've got all the gear their main needs - but there is a slight element of preaching to the converted here as everyone that's commented so far has signed up as often as they can anyway
Personally i'm also not a fan of trying for RS25 (supposed LK25 difficulty although likely a bit easier) while we've still got stuff as easy as valithria up in icc25, but i can understand that people might want a change of scenery.