ML are an easy way to stretch models out, without increasing financial costs. I play against a lot of tough stuff. Necrons, Knights, Marines... there's always a target for a Lascannon.
When the
ML is the same points as a Lascannon, it's hard to justify from a crunch perspective. If you're playing to have peak efficiency,
ML are at least 1/6 less awesome, most of the time. They look good, can be fluffy from a modelling perspective if you're aiming for a "mobile" look to your force. You don't need a tripod and battery pack for the lascannon, just a dude to carry the tube and another guy to lug the rockets, right? More fluffy choice for light infantry.
Rules may change in the future. Who knows? I built Mortar teams when they were hot garbage, and now look at them.
If you like taking swarms of dudes, having the extra
ML's lying around lets you put a
HWT in a squad... and if your people are down with proxies, make a decent stand-in for a Lascannon. Super-Krak missiles... right?
If you have the spare parts, I'd say build them. It can be rewarding in it's own right. Maybe use them as a distraction Carnifex? Maybe 9th edition makes them awesome? Who knows!