I like buying skins that look good on champions. I don't play much with Rumble but his mecha skin is just awesome so I just had to buy it. Although it's a shame he isn't on the meta right now. Sion's new project skin is also looking cool, but haven't played with him once so I'll probably pass this time on it and just wait for the offers.
Interesting question. Here's my situation. I do play LoL a lot. I mean 5 hours a day. But I don't waste my money on skins and stuff like that! I think that is wasting money!