Why even have a trading deadline?

My position is that trades during the regular season should be banned.  But if they are allowed, why not allow them during the entire regular season?  Why not also in the post season tournament?  Otherwise, whatever reasons for restricting when trades may be made should apply all the time.

Trade Deadlines and Other Rules no publication date

by Cliff Blau

Trades are not forbidden after trading deadlines. Rather, all players involved must first clear waivers in order to be traded...

Following the 1922 season, the deadline was changed to June 15, where it remained for decades...

In 1986, the distinction between intraleague and interleague trades was eliminated, and the waivers were required for trades only from August 1 to the end of the season.
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No trades during the season. Thursday, July 9, 2015

I don't know how I could state this any more simply or directly.  There should be no trades during the season.  Put another way: trading should end the day before opening day.  Trading may begin again during the next off season.  The more anal among you may try to parse some ambiguity in this but there is none.  No trades during the season.
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Revolving door rosters on steroids. Tuesday, July 28, 2015

I just read that Troy Tulowitzki has been traded from Colorado to Toronto. That might be good for Toronto but I'm not sure it's good for Tulo, Colorado or baseball.
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Anatomy of a trade: Roger Maris: KC Athletics to NY Yankees. Tuesday, July 7, 2015

Woodie Held and Vic Power were traded by the Athletics to the Cleveland Indians for Roger Maris.  Cleveland GM Frank Lane also traded Rocky Colavito:

• April 17, 1960: Traded by the Cleveland Indians to the Detroit Tigers for Harvey Kuenn.

Cleveland could have kept Maris and Colavito but violated the adage: the best deals are the ones you don't make.
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Rocky Colavito, Baseball Digest, front cover, September 1959 via Wikimedia Commons

The Colavito - Kuenn trade occurred two days before the season opener, which happened to be between the two teams.

Tuesday, April 19, 1960, Cleveland Stadium
Attendance: 52,756, Time of Game: 4:54
Tigers 4, Indians 2 in 15 innings

I wonder how many of those 52,756 fans in Cleveland bought tickets before the trade because they wanted to see their favorite Indian player, Rocky Colavito, who had tied Harmon Killebrew of the Washington Senators for most home runs in the American League in 1959: 42 each. Instead they had Kuenn who had the highest AL batting average in 1959.