A strict comparison of 252 Def / 252 SpD against 252 HP / 128 Def / 128 SpD will show that for the vast majority of Pokemon, it is an advantage to max HP EVs. However, this does not apply to the same degree for all Pokemon, and for some with extremely uneven HP and Def/SpD base stats, it is indeed better to max both defensive stats than to use any HP. For example, Wailord's very high base 170 HP and very low base 45 defences mean that 252 Def will give it better protection against physical attacks than 252 HP / 128 Def. In general, the higher the Pokemon's HP is compared to its defensive stat, the more worthwhile it is to use EVs in Def and/or SpD -- it just takes a massive outlier like Wailord for it to be *better* outright. The same correlation applies for Pokemon like Shuckle, whose HP is rubbish in comparison to its defences and is therefore much better off investing HP before any defensive stat.
All of the above is pure maths though, and none of it is to say that you shouldn't use HP EVs even in extreme cases like Wailord's. If you have an 'outlier' Pokemon that values extra protection in one defensive stat a lot more than the other (e.g. because one stat is particularly weak), then they might run 252 HP anyway, because it will help with that important stat. An example is Chansey, whose uneven HP and Def clearly fits the bill -- except it will always use 252 HP / 252 Def, because it is already excellent at tanking special attacks.
A long winded response, but hopefully it makes sense. The short answer is you should only use 252 Def / 252 SpD when your Pokemon's HP is much, much better than both its Def and SpD and the Pokemon values its Def and SpD similarly. You can play around with a calculator like this if you're still curious:
https://calc.pokemonshowdown.com/
