Well, the MD-90 isn't a half bad, it only has four major disadvantages:
1) It only comes in one size, so the only way to upgauge a route is an additional frequency. 737 and A320 both come in different sizes and can be switched for a larger aircraft without even having to adjust schedules.
2) The MD-90 isn't as sought after as the other types. This has the advantage of it being more readily available now, but it can turn nasty when the production line closes (may or may not happen) and you want to expand your fleet.
3) It has shorter legs than the other types.
4) It is slightly slower than the others.
If these are non-issues to you, the MD-90 might even be the ideal aircraft. Oh, and if I was you, I would replace my 717 and 737Classic fleets anyway, most 717 routes can also be done with an MD-90 and some less dense ones are probably not making enough profit to warrant for an additional fleet type anyway.
The 737NG and the A320 in particular win in about every single category (if only slightly), but they have the problem of not being easy to lay hands on.