Part of the issue with this "ignore errands to focus on important things" argument is that often times ignoring errands makes it harder to focus on important things. Kinda hard to get into a flow state when you're homeless and starving, for example, so you're inevitably going to need to do at least what's necessary to not be homeless and starving. That is: while you'll very likely never be memorialized for all those reports you generated or whatever as some faceless cog in a corporate machine, you still need to do them - or else you'll be in an even worse position to do the things for which you actually
would be memorialized.
(At least until we achieve fully automated luxury gay space communism, but in the meantime we're subject to the material conditions in which we live - and those conditions, at least for those of us in the working class, currently necessitate doing work which is unimportant in the grand scheme of things but pays us money with which we support our existences)
There's also the issue of importance being arbitrary and relative. For example, I created a scripting language for Erlang/OTP applications. If you're looking for a scripting language for your Erlang/OTP applications (and you happen to think that Tcl's syntax is the best thing since sliced bread, and can tolerate the fact that my work is very unfinished), that might be very important to you. To the vast majority who don't know what the actual fuck "scripting language", "Erlang", "OTP", and/or "application" mean, that's almost certainly entirely unimportant to them. What's (possibly, at risk of over-inflating my ego) revolutionary in my niche is entirely irrelevant to how nearly everyone around me will know me beyond "that YellowApple guy was pretty good with computers". Is that niche accomplishment really more important than how I treat those around me? Would I rather be remembered as "some reclusive asshole who was smart with computers" or "a beloved friend who put others before himself"? The example about writing letters is perfect; I procrastinate on responding to texts/calls/etc. from friends and family all the time, and I absolutely hate myself for it because I do love my friends and family and it hurts me to know that I'm hurting them by being an unreliable shut-in.
For some of us - hell, probably most of us - the "biggest thing" on which we can work is being the best human we can be to those around us. It's easy to forget that in a ruthlessly capitalist society like the one in which I and most other folks live (in general, let alone the subset reading Paul Graham's blog posts). Sometimes being that best human means sacrificing personal relationships for some revolutionary thing that'll improve the lives of hundreds or thousands or millions or even billions of people. Other times it means forgiving interruptions by loved ones, and treasuring those human connections over whatever mental state you just lost. Sometimes that might even mean figuring out how to reconcile the two, e.g. by writing down your thoughts as they occur so that interruptions by loved ones are less catastrophic to your workflow. What matters is whether what we do maximizes the positive impact on the world around us, and for most of us that maximization is realistically more likely to happen by supporting those around us and fostering mutually-healthy human relationships than discarding them in favor of self-important endeavors.
Put more succinctly: determining whether procrastination is "good" or "bad" is doomed to inaccuracy if we're unable to examine ourselves from the outside-in and put our own notions of what's important into perspective - and irrelevant if we're unable to fulfill the "unimportant" dependencies of human existence necessary to accomplish those important things.
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"Ye are the light of the world, a city set upon a mount is not able to be hid; nor do they light a lamp, and put it under the measure, but on the lamp-stand, and it shineth to all those in the house; so let your light shine before men, that they may see your good works, and may glorify your Father who [is] in the heavens." -- Matthew 5:14-16, YLT
I know not everyone here puts much stock into Bible quotes, but this one comes to mind after reading the essay and writing out my thoughts above. The important things (good works) are those which positively impact (shine light before) as many as possible, to as high of a degree as possible. You don't need to believe in some sky wizard to see the value of being remembered as a good person who left the world better than one found it.