As far as I see it, perhaps the question should be, why not take a positional relay?
After all, if you know your unit is very likely to come in first time, deeepstriking a single unit becomes a much more realiable tactic that opens up opporunities you might not have risked before.
Similarly, if you have it in your army but decide you want to deepstrike everything presumably you don't have to use the positional relay?
Whilst thirdly, if you find yourself playing a drop pod army, that positional relay is golden because you can delay your army coming in as has been pointed out.
In a tournament list, it seems like something that will very often be not so bad, very rarely will it be completely useless (even if you don't eventually use it, having the option might be worth the points) whilst in some games like drop pods it could be worth many times its points value.
So, unless it's rediculously expensive (I have to admit, I don't know how much we're talking here points wise) it seems to be something worth having just as a card up your sleeve.