My main premise is that the notion of a true "rental" is a bad one. You should not play most of the season with the roster you built and then magically become a different team for the last couple months. It's the equivalent of bringing in a "ringer" only you pay something for the ringer. So I guess it's like paying a ringer $50 to show up at your team's softball game for the last 2 games of the season. "Oh, our teams are about even? Well, not the last 2 games they won't be because now our team has a new cleanup hitter."
I think if you are going to add key players to your 2014 team, they should be on the 2014 team for at least half the season so that they are actually part of the core of your team that season. I feel the trade deadline should be June 30th and then the waiver deal deadline can be July 31st.
In this scenario it becomes more of a seller's market because buyers are getting more value for their acquisitions. Specifically, for a rental they are getting the player for 3 months, not 2 months. That's fine. I see no reason why it's imperative to make the market any more of a buyer's one or keep it any less of a seller's one. It is what it is, take it or leave it. You're supposed to build your team primarily in the off-season anyway.
What's that, you say? (Or maybe it's just the voices in my head.) Teams don't know by the end of June if they are buyers or sellers? Too many teams are still alive in the wild card? Tough. Billy Beane traded Joe Blanton and Rich Harden in late July when the 2008 A's were just a few games back in the division, because looking at a combination of advanced metrics, common sense, and his favorite ouija board, he forecast that the A's were going to heavily regress with or without 2 of their key starting pitchers. And that's why the A's now have Josh Donaldson.
Anyway, the current system promotes too much of the concept of bringing in a ringer for my taste. I don't think the Angels should suddenly be able to say, in August and September, "Oh, our rotation is better than you thought. You see, it actually has David Price in it." And if the A's wish to turn Eric Sogard, despite his undervalued accounting and cookie baking skills, into Chase Utley or Ben Zobrist Daniel Murphy, then that "real A's team" should be assembled for half the season if it's going to be assembled at all.
To me it's just the way rosters and "who your team is" should work. Your thoughts? No hurry; there's no deadline.