Currently you can get One Day of Protection for 100 crystals but if you attack another base you lost that protection even though you paid hard currency for it.
100 Crystals / 24 hours is about 4.16 Crystals an hour. If the game offered 1 hour of protection for every 5 crystals then it is worth it because you can protect your base while preparing for the next attack. Air support can sometimes take 1 to 2 hours. Droidekas take 1 hour +.
The player would be able to protect their increasing wealth and Disney would cash in on more use of crystals. Win-Win situation.