I have five DLE engines and always use the spring. I've flown thousands of flights and never had a problem with them. When the 120 degree caps first came out they were hard for people to get on and I think they blamed it on the spring. The loose caps caused a lot of problems. Most people have learned to make sure the caps are tight now.
The spring serves two functions. One is to prevent the ignition from interfering with your receiver if the cap is loose on the plug, that can cause possible loss of control. The other is to insure that the cap makes a good electrical ground contact with the plug during engine vibration. If the contact is loose or intermittent, the ignition module or the hall sensor can be damaged.