Use Toxic Spikes, when you
*have trouble with bulky ground and/or water types and especially Blissey
*especially rely on special attackers, as your main counters are destroyed by Toxic Spikes
Even when your opponent isn't affected much (most times at least one Pokémon is affected) Toxic Spikes is a good addition to all teams having Empoleon, or other special sweepers, since it makes their job 1000 times easier.
Maybe there are few Pokémon that Toxic Spikes really hits, but those defensive, nasty threats:
I'm using the October usage list to show them:
#7 - Tyranitar - Rest isn't used enough to be mentioned (Other: 7,3%)
#8 - Infernape
#12 - Swampert - Rest isn't used enough to be mentioned (Other: 7,9%)
#14 - Blissey - Aromatherapy (27,4%) ; Heal Bell isn't used enough
#15 - Starmie - Rapid Spin (54,5% + Natural Cure)
#18 - Machamp - Guts (9,4%) ; Rest (25,3%)
#19 - Vaporeon - Rest isn't used enough to be mentioned (Other: 5,6%)
#22 - Celebi - Rest and Heal Bell aren't used enough (Other: 9,2%)
#28 - Jolteon - Rest isn't used enough (Other: 3,4%)
#29 - Kingdra - Rest isn't used enough (Other: 7,8%)
Yes, only 10 of the 30 most used Pokémon are affected by Toxic Spikes, but you have big defensive threats covered. Though Blissey just can switch out and cure it's poison by Natural Cure, Blissey can easily be stalled out and can't perform her job as a special wall. She also gets in danger to fall to strong special special moves, because strong special moves + toxic damage can actually kill Blissey easily in three turns.
Not enough bulky grounds and waters run Rest (see above), so they become USELESS, because of Toxic Spikes. It also makes Tyranitar look pretty bad.
There are a lot of more Pokémon affected badly by Toxic Spikes. Suicune (only 50% run Rest); Electivire, Weavile, Hippowdon, Snorlax (17% Immunity; 63,5% Rest), Dusknoir, Mamoswine, Smeargle, PorygonZ, Heracross (attention: Guts) are the other OU-Pokémon that are affected by Toxic Spikes.
Now, look at the counters:
-Roserade (#40 - 4,53%) and Tentacruel (#41 - 4,5%) absorb them by switching in.
-Rapid Spin users (alphabetic order, over 1% usage in OU):
Claydol: 1,6% - (77,8% RS)
Donphan: 2.3% - (84,8% RS)
Forretress: 7,3% - (82,6% RS)
Hitmontop: 1,6% - (46,8% RS)
Kabutops: 1,5% - (7,9% RS)
Starmie: 11,7% - (54,5% RS)
Tentacruel: 4,5% - (89% RS)
Actually we have only three OU-RS-users: Tentacruel, Forretress and Starmie and it's not like that you can't switch into a ghost. Also, Toxic Spikes aren't bad, just because they can be removed by RS, so you wouldn't lay down any entry hazards and we all know how important SR is in the metagame. The far worse thing is that Toxic Spikes are removed by switching in grounded poison types, so you really need pay attention to them.
The biggest issue is the following: There are millions of SR-users, but there are very few Pokémon that can use TS. In my opinion it isn't quite hard to fit them in your team:
-Forretress, especially with the mentioned moveset above is quite awesome
-Roserade as status user, annoyer/staller (with leech seed), small special tank, lead or special attacker.
-Tentacruel, as a special tank and RS-user
(and maybe Smeargle....)
It's important to say, that Toxic Spikes are not very helpful for every team, especially when you rely on other status as paralysis or want to abuse Yawn.
=> In my opinion Toxic Spikes are very usefull. As you can see above Toxic Spikes are very usefull, when you have trouble with bulky grounds and bulky waters. When you are relying on special sweepers, Toxic Spikes supports you, as it makes the job of many Pokémon that can be difficult to kill (!Blissey!).
Instead of saying how many Pokémon aren't affected by Toxic Spikes and how useless it is against the majority of the OU-Pokémon, you should note how many threats are destroyed and are becoming useless just because of Toxic Spikes. Although most of those Pokémon can use Rest, abuse Natural Cure, or use Heal Bell / Aromatherapy it isn't used that much. Just look at the statistic.
-0²