Could get it as a testing/backup PSU if the decent one goes mind
Sounds sensible, using a PSU that's known for being pretty bad as a backup.
Seriously, using the ZS 550 as an example (£54.98):
Antec HCG 400 is £11 cheaper, and much better. Provides around 50W less.
Antec TPN 550 is £5 more, provides around 100W more power and is better quality.
Corsair CX 430 V2 is £17 cheaper, best choice for a backup PSU though I wouldn't recommend it as a main PSU.
Lepa N series 600W is £11 cheaper, around the same quality and provides more power.
Lepa W series 450W is better quality and £11 cheaper, provides the same amount of power.
Lepa W series 500W is better quality, £8 cheaper and provides more power.
XFX Pro 450W is £9 cheaper, better quality and provides the same amount of power. This would be my choice if I wanted a good low power unit.
The same holds true for the "higher output" ZS units, there's something better and cheaper at every level.
Good deal if you
really want MW3 and don't care about your PSU or want to sell it to someone who doesn''t care about theirs.