Sure I like it better when they print the whole thing, but the goal is to get the message out. Strunk teaches the importance of brevity. Strunk's terse lessons on omitting needless words and using the active voice do more to improve most writing than any single thing I know of. Strunk understood the fundamental proposition that less is more when it comes to effective writing. The fewer words you use the better.
Wordiness weakens writing.
Simplicity has the power to change minds and change the world. Short and sweet is essential for Letters to the Editor because your letter will only be one of hundreds on every subject under the sun competing for very limited space. So you know up front that a long letter has very little chance of being published. The only really long letters I see come from government officials, heads of large corporations, celebrities, leaders of organizations, the target of an article and the like. Most were half that length.