Sentiment aside, canteen cups have two drawbacks which can be remedied with products now on the market.
|Nothing like hot chocolate from your canteen cup.|
First is a lid. The US military never issued one. If you're heating something by putting your canteen cup in a campfire you don't really care too much about minimizing fuel consumption, but it's nice to reduce heating time and keep debris out of your meal. Previously I've used a piece of heavy duty aluminum foil but this needs to be periodically replaced, and it's prone to blowing away. Heavy Cover, Inc. sells a nice stainless steel boil lid, shown in the above pic. HC also sells one for the Crusader cup.
I've used the Heavy Cover lid a couple times now and it definitely reduces the time to boil water for a hot drink by a few minutes. If I have any criticism is that it's heavy. It could be made from a thinner gauge of stainless steel to reduce weight. Or better yet, make it from aluminum or titanium. E.g., Four Dog Stoves sells Ti lids for a variety of pot/cups, for $4 less than the HC stainless steel lid.
Second is something to prevent burning your lips on the hot rim of the cup. Snow Peak Hotlips happen to snap right onto the USGI cup like they were made for it. The yellow thing on the rim of my cup above is one. This is a major usability improvement when making coffee, tea, or hot chocolate.