If you're on a budget, you could certainly relegate the old speakers to surround-channel duties. Depending on how tonally coloured they are, you may notice the differences in coloration between your main speakers and the surrounds on some material.
Working in your favour is the fact that our ears are remarkably accommodating to a speaker's tonal "character" and after listening for a few days, many enthusiasts come to accept the sound of their speakers--even though they may be fairly awful--as their "reference." Only in a careful, instantaneous comparison (preferably "blind") will the older speaker's flaws become cruelly apparent. Then again, it may have been a fairly neutral speaker. No harm in trying them out in surround roles until you're ready to upgrade.
Axiom Resident Expert