It takes seconds, seriously! And it's easier just to reply straightaway than to have to remember to do it later.
I texted a friend of mine re meeting up in a few weeks. 48hrs later - no response. Now, meantime, I'm having to keep that day free for her!! Now I understand she might be checking her diary or whatever but cldnt she text "checking diary will get back to you". Instead of me having to wonder "should I check to see she got the text or is that going to annoy her?"
FWIW she has no DCs.