Best tool for the job. Doesn't mean that people should plan to replace photoshop with mspaint, but it also doesn't mean that mspaint shouldn't be available.
Best tool for large spreadsheets, documents, etc: still a PC (or a laptop).
Best tool when one is on the go: Might not be the above. Just like a camera on a cellphone isn't going to compare to a DSLR, it's still a useful thing to have for many people depending on the situation. Not many people cart a full-sized camera around all the time.
I don't plan on writing out a master's thesis or a 1000x1000 cell Excel spreadsheet on a tablet, but for many more casual uses it's still useful.
Nobody is telling people that they *have* to use tablets instead of PC's (well, MS did foist a crappy tablet-centric interface out with win8, but different topic). They're saying the option is available.