Yep, it's a good idea which makes teleporters slightly easier to maintain but isn't a huge balance issue considering the recently introduced ability of the Spy to take enemy teleporters. Don't think how close your Engineer is to your sappers, think how close YOU are to the engineer.
If you're at the exit, the engineer is probably close, ready for stabbing. If you're at the entrance, you can just take it and still end up close to the engineer.
Before the change teleporters were a pain in the ass to build and maintain, now they actually make sense.
Build teleporter entrance, teleporter exit, upgrade them - *zap* a spawn-camping spy just nullified two minutes of your gameplay time and forced you to go back to the spawn and running back and forth is exactly what teleporters are supposed to help alleviate. Not to mention the fact that when you abandon your sentry/dispenser THEY will get destroyed in turn, which is even more annoying and harmful to your team, so in the end why bother building teleporters at all?

I was referring mores specifically to, I stab engi, sap stuff. Other engi at spawn removes sapper the whole flipping time. Or run into a non defended sentry emplacement sap all stuff. Engi wrenches his teleport at spawn, ports through and unsaps everything. Its retarded and already happed to me a couple of times. I'd be fine with the change if it only worked if the spy saps the entrance. But the sapper should not teleport through an exit.