You only ever have 3 refunds for your account, and in the long term you're going to have plenty of IP (and ultimately want all champs anyway, if not to play yourself, then to have to trade around in champ select). You're going to be playing the game regardless of if you're intentionally trying to get ip or not, and hence it'll always be coming in, so you're eventually going to have a surplus. If you spend IP on something you decide you don't really want, then it just delays your next purchase by a little bit, but theres no lasting impact.
RP on the other hand you can't normally earn through natural play, and hence costs you actual real money. If you buy something with RP and decide you don't actually want it, then you're just screwed out of that RP(/real money) and can't get it back unless you have a refund token.
As a result, many people feel you should probably never use refund tokens except on RP purchases, since it feels a lot worse to lose real money than IP if down the road you're out of refund tokens.