Actually if you do the math killing 1 fusion is usually not worth it at all. assume the enemy reclaims.
eco player lose a fusion (-1000m, -1000e, -1000bp), but gain a jack wreck (+240m, -240bp), a fusion wreck (-400m, -400bp) and a swift scrap (+30m, -30bp)
1670 bp used
jack yeeter lose a jack (-600m, -600e, -600bp) and a swift (scouting, -150m, 150e, 150bp)
eco player comes out 420m on top
jack yeeter comes out 250e on top
jack yeeter comes out 920 bp on top
now time to translate E, bp and other stuff into metal.
let's assume a overdrive rate of 8e -> 1m (which is the point the game tells you to stop being a lobster and build more mexes). 250e translates to 31.25m
at a 8e -> 1m convertion rate, a fusion would take a little under 4 minutes to pay itself back. so lets assume your cons (which will be the generic-ish 130m cost 5 bp) also need to pay themselves back in 4mins, and that half their time is spent walking. that gives us aprox 0.216 m/e per bp, or 0.243m per bp, or aprox. 224m for the whole 920 bp.
lets say it takes 30s for for the fusion to get back up again. this will mean 1050 lost E and therefore 131.25 lost M
so all in all, the jack yeeter loses cost even in a successful hit. if they managed to kill more stuff they might be able to make even, but remember that the gotta make back the full cost of their ramp in 4 minutes and keep the rate up or else a singu would've been a better choice.
TL:DR - Singus or mohos are usually better than jack ramp.
EDIT: forgot to take into accout the cost of the BP needed to build the cons or caretakers having better BP rates than regular cons but should've really matter that much.