If you gather them yourself, it is worth the your time. You wil get some gil, nice and nice items.
Even if you have no use for the items yourself you can sell most of them, which can net you even more gil.
If you plan on buying them it is probably a waste of gil.
The only exception here is if you are doing peisteskin maps with a group. In that case you can buy them for up to 10,000 - 12,000 gil and still come out with a profit.
I usually grab a map each day. Currently boarskin maps because I want a tonberry minion, and after that I will return to peisteskin maps for fc events. You can have one map of each type at the same time. BUT, you can have a deciphered map and an undciphered map of the same type at the same time. (IE: one deciphered peisteskin map, and another undeciphered one). You can also entrust them to your retainers. Those will also only be able to hold one of each type, but this allows you to stock up on maps and do multiple maps later on.
One last note: you cannot gather a map if you already have an undeciphered map of the same type in your inventory. But this does not mean that you lose your chance to get a map that day. This is especially hilarious with fishing, as you can keep fishing up maps and the game keeps telling you that you can't obtain another map (this counts towards the map challenge in your challenge log btw, so you ca get the "gather 3 timeworn maps" challenge in a matter of minutes)
