[custom-elements] Rationale behind 5.2 (copying properties of the last value to a new prototype)

Hello,

in 5.2 "Methods and properties" [1] the Web Components draft states that:
"The properties of the last value of the script will be copied to a
prototype object created for you.".

What was the rationale behind choosing this design over more explicit one
(i.e. this.tick = function () {...})?

Thanks,
Kamil

[1]
http://www.w3.org/TR/2013/WD-components-intro-20130606/#methods-and-properties