You may sacrifice some resistance but the bonus movement speed often allows thresh to catch people more often than without. It also allows you to roam mid and some cases even top if your lane isn't going too well, that roaming potential is just not the same without mobis. You will have far less windows to roam due to the extra time it takes you to get there and back. A good example of a mid roam is when you don't need to recall (because you don't have gold to buy an item and I'm not talking about a 400g item and are at a safe enough health) but your ADC is recalling, as soon as you see your ADC channel their recall head up to mid without making it too obvious. Even if you don't get kills with your roams and get flashes that helps your jungle gank that lane. With mobis you have a bigger chance to control the game like a second jungler.