Why buy 5 sets of Lockhart's book?

[romuluslupin1]

No: HPFGUIDX 113614

I was watching COS again yesterday and a thought struck me. Why did 
the Weasleys, who are reputedly so poor, spend so much money on 5 
sets of Lockhart's books. Even with Harry giving Ginny his free 
copies (or is this movie contamination?) they must have dug a large 
hole in the family finances. Wouldn't it have been cheaper to buy a 
couple of sets and have the kids swap them on a need to use basis? 
After all they have DADA lessons at different times (well, of course 
Fred and George don't) and we saw from Harry's notes on FB that 
books can be shared.

Romulus Lupin, who loves Gilderoy's Italian name better than 
Lockhart (In Italian his name is Gilderoy Allock, which is an 
anglicization of allocco, a name reserved for barn owls and morons, 
which Gideroy definitely is)