ginti
Contributor
At what point does one need to buy a primary. .?
I mean, it really depends on your needs.
Rec diving, only during the day? No need for any light (although I always bring it with me, sometimes you can use it to see inside cavities and holes)
Cave diving? Well, you better have three lights with you; a primary and two backups. If you don't need long battery time (that is, intro to cave level), you may want to opt for a cordless light.
Deepish deco diving? Maybe a primary and a backup; in this case, I prefer a primary with a cord even for short dives (if you drop the light by mistake, it will not fall 40 meters deeper than you - that is, you are not going to lose it)
Night diving? Again, at least a primary and a backup; same consideration for the tec-dives.
If you do dives that require lights - you better buy them.
If you plan to do dives that require lights - you can evaluate to buy them, depending on when you will do this dives and your budget****.
If you do NOT do these dives, and you need a light just for the course - rent the light, don't waste your money.
Again, this is not true if your goal is to get a tec-pass; in this case, even for the course, I believe you need to own the equipment, so check it with your instructor. But if you are going for a tec pass, most likely you are interested in tec/caves so that you will need your lights very soon - in other words, it's the time to buy; the question is: what to buy?
****I believe that the more you delay a purchase, the more you will know when you make the purchase; this is why I always suggest to take time before to buy something, except if you have a deal in front of you... but other people may think differently