To me Lotus has always been a no brainer.
Game after game I keep repeating "lotus in base" and nubs never listen, and get their base destroyed by a single pyro.
I'll tell you why I use Lotus:
1) It's very efficient. For a measly 90 metal you get a static unit with good damage and a decent range, that can be put behind tough buildings like solars to enhance its effectiveness.
2) Lotus secures your expansion. There's nothing worse than grinding forward and suddenly your big heavies get cut down by a bunch of glaives and you get chased back to your starting position. Securing the advance with lines of lotus provides not only a fallback line but also hampers light unit swarms from simply screaming back up into your territory in a counterattack.
3) Lotus is good for porc push. Because they pop up so fast you can even advance against pickets with them by building them inside the picket radius of fire. I've often had commander vs commander fights determined by my popping lotus up faster.
4) There seems to be a sudden prevelance for using swarms of fleas again. A pair of lotus can eat a bunch of fleas for breakfast and they nicely complement ranged units that find themsleves swarmed by them. In fact ranged units + lotus are quite effective I find.
5) Lotus works well with other statics like faraday. A couple of faraday become a lethal obstacle with the addition of 2 lotus. 180 extra metal buys you an awful lot of protection against base drops and short ranged units.
Finally: Lotus can help fight air units to some extent. Stuff like Locust becomes much less effective at raiding when Lotus are dotting bases, and gnats can be killed off before comnappings are completed.
TLDR: Lotus is very efficient at many roles and costs little. Build lotus.