Only if you can't earn enough miles other ways and if you don't need them for hotels.

Hotel points, when used judiciously (ie, not in all circumstances) can be worth more as such than when transferred to airline miles. But if you find you never need hotel points (perhaps because you stay in other people's homes when traveling rather than in hotels, for example), then it may be reasonable to earn miles.

But have you already maxed out on airline credit cards? You don't tend to get huge points bonuses with hotel cards (once you factor in the hotel:airline conversion) the way you do with some airline cards (or even credit cards points cards that can transfer to airline miles in the case of UA). For example, the Amex SPG card only gets you 10K points (10K to 12K miles) "easy" and then only an addtional 15k points (for a total of 25k to 30k miles) with a high spend within a few months.

Compared to that, if you haven't yet (within the last 18 to 20 months), you can get 2 Citi AA cards (one personal Visa, one personal Amex) to 90k to 100k (depending on the exact offer), with no more spend needed total on 2 cards than on the 1 SPG Amex I think.

Then with UA, you can get a Freedom for 30k Ultimate Rewards points bonus, then a Sapphire Preferred for 50k UR points points plus that Sapphire Preferred makes it possible to transfer all your UR points to UA 1:1. And on top that you can also get a UA card itself, for another 40k or more bonus!

Compared to that, SPG is one-time 10k to 25k points bonus, and that's it. Ever. The one advantage of SPG, beyond the bonus, is that it has a 20% transfer bonus for transferring to AA and DL. But since it transfers to UA at a lousy rate (2 points for only 1 mile), I wouldn't recommend it for UA earning.

I don't understand why you want a hotel card instead of airline cards or Ultimate Rewards cards for ailrine miles. It's only the SPG Amex, no other, that gives you that 1.2 miles per dollar spent (in the long run); all other hotel cards give you less than 1 mile per dollar spent.