Bulbs and sensors work differently in this sense.
The bulbs should always be used with the bridge for the reasons above.
However, the bridge does not expose the “accessories“ like sensors and switches to smartthings. It could, because quite a few of them are exposed to HomeKit, it just doesn’t. I’m not sure why. The integration was originally built by smartthings engineers, not hue engineers, and I don’t know if that made a difference.
There are a couple of options for the sensors. Since the sensors are not repeaters, you don’t run into the issues that you do with a directly connected bulb. That’s the good news. 
A) if you just want to use the Hue motion sensor with smartthings and you’re not trying to also see it in the Hue app, then there is a community created DTH that will work with it. This is a popular method for use with the outdoor version of the sensor.
B) if you want to use the Hue motion sensor with the bridge and also use it with smartthings then there are a number of community developed options, although they all have some limitations.
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If you happen to use iOS and you want to also use homebridge, you can bring your smartthings devices into HomeKit and use the sensors that way. But that’s a long way to go for an integration.
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The sensors can be used to trigger an Amazon Alexa routine (not a smartthings routine) and you could have that turn on a virtual switch to act as a proxy for the sensor. That’s a lot easier to set up than homebridge and it works quite well in this direction. It does make you dependent on two different clouds (Amazon and smartthings) but it’s not a bad way to go. You don’t even need to have an echo device, just an Amazon account and the free Alexa app.
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you can use IFTTT, although since that’s now only free for three applets, it’s not as appealing as it used to be. and again, two clouds.
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if the use case allows, you can have the hue sensor turn on a specific Hue bulb via the hue bridge, and then use that bulb coming on as your trigger. This can work for some use cases, I did something similar at my own house for a while to turn on a desk lamp and then have that lamp coming on trigger the overhead light, but it won’t work for all use cases.
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There is a community created custom code version which gives you access to information from a bridge connected sensor, but some people have reported it was not reliable for them. And, as you noted, you have to have at least one bulb on the bridge in order for it to work. You can talk to people using that method in the following thread: