Well, they give you 5 more mana, so eventually you will reach 15, but it is much harder to get CCC in play and you are less flexible in the early to mid-turns.
t1. Plains (W)
t2. Azorius Chancery, bounce Plains (W)
t3. Plains (WWU)
t4. Selesnya Sanctuary, bounce Plains (WWU)
t5. Plains (GWWWU)...