Walls shouldn't be focused on coverage... Leech Seed does much more damage in the long run through a battle, while Hidden Power is only useful against certain Pokemon. The lack of Leech Seed doesn't really make Tangela worse or better as a wall, on one hang it has a tad more trouble with certain threats and on the other hand, it has extra recovery and can help heal its teammates.
But a lack of coverage hinders its ability as a wall, besides, if you can constantly force out Mag or at least accumulate damage on it, it helps a ton, given that Magmortar is one of the best counters to Tangela and is threatening in general.
Right, but Leech Seed saps their health well enough to. In fact, HP Rock only does like 35% to Magmortar. Tangela can Leech Seed one of those 2 on the switch then switch to the appropriate counter who also get some health back! It's a lot more useful on a more defensive team, but it can still be effective on other teams.
Hi DTC, to respond to your Tangela post, Leech Seed is adorable and all, but Tangela loves to accumulate that extra damage to Swellow/Mag with HP Rock when they switch in and take SR Damage. In addition, Tangela can just switch in and out to get back its health and just whittle down the opponent's Pokemon's health with Giga/HP. HP Rock and Ice are extremely underrated in what they can beat, whereas Tangela doesn't have to independently rely on leech seed to take down most mons, where it does to take out something like Altaria with HP Ice.