Its not as helpful as it looks - you wouldn't start an invasion this way - by drip feeding units across. You would use every available transport and stuff it with troops. Your second wave of reinforcements won't be sent this way either - they will probably be sent on transports that were constructed just after the invasion fleet was sent and dispatched as soon as they can - long before you could establish the chain.
Later on it could help accelerate a third and subsequent wave, but by then you probably have a secure foothold and will want to use your transports for rapid assault up the coast rather than leave them in a long hard to defend chain.
In general the success of an intercontinental invasion for me isn't in how fast I can get reinforcements there - its whether I can arrive in sufficient force to establish a strong beachhead and pursue further cities. This strategy doesn't help me there and it carries a risk in making my transports hard to defend.
So I agree - its probably an exploit but I am not sure it helps you much.