I just stated PL maps as an example of what can happen when an engineer is supported. The reason for this is because of how the maps play, a good team will work together to protect the sentries and a bad one will not, and suffer because of it.
And as far as most maps being full of choke points, that is so the engineer can use his buildings. On an open map, the engineer is next to useless in the open unless he is supported by sniper/soldiers to counter the enemy's snipers/soldiers. A good team will cover choke points with more than one sentry, just look at 2fort if you need an example. The engineers will usually cover the entrance to their base with at least 2 sentries if it is a full server. The engineer is designed as area denial, and he does that well enough already. If you give him more toys, he will become like the dead ringer spies or the FaN scouts, a huge annoyance due to how unbalanced they are in the hands of good players.
The point of my argument is this: How can you really make the engineer better, yet not overpowered? The simple answer is that it is impossible due to how strong, but balanced he is right now. Therefore the engineer does not really need anything. The only reason for this update is because Valve updated every other class, and they feel obligated to update the engineer because of this.
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