User blog comment:Alartan/Swiftness or Mobility boots?/@comment-5266525-20150201191955/@comment-4022742-20150205140149

Using BoM for engages is not only risky but also a tad counter productive, and not what they are meant for at all. Sure you can move fast and get in a good position with those boots, but hitting anything or getting hit kills your advantage. On top of moving faster then all of your allies who might not even be able to keep up and take advantage of your epic engage.

Boots of mobility should be used for level 6+ roaming, getting from 1 spot to the other faster is a huge advantage in this stage because there aren't a lot of wards at this stage yet. Later on it falls off a bit, mainly because proper warding will result in the enemy seeing you coming.

A better option to use for engages, more reliably too, is Righteous Glory. Not only do you move faster, so do your allies, with a bonus AoE slow at the end.

Wether you decide to go for mercs, tabi, or swiftness depends on their CC and damage type.