I'm not on the PBE for the free RP/IP, but I do like to save up my IP so if I ever want to experiment with a rune combo I don't have on a new champ I can just snatch up what I need if I don't have it. Same goes for RP. Sometimes to test new changes to a champ on PBE, I have to buy them. Going to be hard to keep doing that is I run out of RP and IP
Also, there is almost no possible way for you to have such a litle amount of RP and IP that you can't test out New Changes to an existing Champ, or even a new champ. All new items (such as Skins, Runes, and even Champs) are all worth 1 - 10 RP/IP for quite a considerable time.
If you don't have 10 IP during a new champ release or skin, then go play a game. Even if you lose, you definitely gain 10 IP.