So my question is "why bother?"

Really, what do you do with this number once you have it? Do you
calculate this so that you can justify an extra Twinkie?

I think information is a good thing. But I also think that endless
amounts of data are a waste of time.

Having a good fix on caloric consumption may be useful at high levels
of competiton for such long distance events as TDF, the Giro, etc. But
what use is it to the recreational rider? Are you really going to
count your calories in vs. out?
I think it's a bit silly. If you have a scale you can easily monitor
1) short term wt loss (from a ride) which is a measure of dehydration
and 2) long term weight change trends which is actual weight loss or
gain. Calculating it on the ride seems to be an effort to have a
number and little more.