  The JWST MIRI instrument will revolutionize extragalactic astronomy with
unprecedented sensitivity and angular resolution in mid-IR. Here, we assess the
potential of MIRI photometry to constrain galaxy properties in the Cosmic
Evolution Early Release Science (CEERS) survey. We derive estimated MIRI fluxes
from the spectral energy distributions (SEDs) of real sources that fall in a
planned MIRI pointing. We also obtain MIRI fluxes for hypothetical AGN-galaxy
mixed models varying the AGN fractional contribution to the total IR luminosity
($\rm frac_{AGN}$). Based on these model fluxes, we simulate CEERS imaging
(3.6-hour exposure) in 6 bands from F770W to F2100W using MIRISIM, and reduce
these data using JWST PIPELINE. We perform PSF-matched photometry with TPHOT,
and fit the source SEDs with X-CIGALE, simultaneously modeling photometric
redshift and other physical properties. Adding the MIRI data, the accuracy of
both redshift and $\rm frac_{AGN}$ is generally improved by factors of $\gtrsim
2$ for all sources at $z\lesssim 3$. Notably, for pure-galaxy inputs ($\rm
frac_{AGN}=0$), the accuracy of $\rm frac_{AGN}$ is improved by $\sim 100$
times thanks to MIRI. The simulated CEERS MIRI data are slightly more sensitive
to AGN detections than the deepest X-ray survey, based on the empirical $L_{\rm
X}$-$L_{\rm 6\mu m}$ relation. Like X-ray observations, MIRI can also be used
to constrain the AGN accretion power (accuracy $\approx 0.3$ dex). Our work
demonstrates that MIRI will be able to place strong constraints on the mid-IR
luminosities from star formation and AGN, and thereby facilitate studies of the
galaxy/AGN co-evolution.
