It is an extremely common leak to complete the small blind too much, and even to defend the small blind against raises too much.
You flop 2 pair or better about 3.5% of the time. When that happens, you have a strong hand, but you might not get paid off, and you might not win. When you have a trash hand, it is not very valuable to flop one pair out of position. It is hard to get paid off by weaker hands, while you may have to pay off stronger ones.
Let's take a hand like A2o. Most bad players will not consider folding it from the small blind. It can easily be the only ace out there, flopping top pair means you don't have to worry about any overcards, A2o is only about a 70-30 underdog to KK, and it can make a straight using both cards. However, in 10-handed limit games, players lose an average of about
0.27 big bets when they are dealt A2o in the small blind, as compared with losing about 0.25 big bets by folding preflop rather than completing the small blind. While there are profitable times to play A2o from the small blind, players would be better off on average folding it every time rather than playing it as they do now. If A2o is often a close fold, then many weaker hands are easy folds.
In limit, whether the small blind is 1/3 or 1/2 or 2/3 of the big blind matters a lot. When it is 2/3, and the big blind is not raising frequently, completing is better than folding even with the worst trash. When the small blind is 1/2, I usually complete with many, but not all suited hands, and many offsuit connectors and 1-gappers. When the small blind is 1/3, and in NL, I require a decent hand to complete after limpers. I usually fold K9o and A7o. The positional disadvantage is large relative to the discount.
A common mistake is to imagine that you are getting good odds in multiway pots. There is a huge difference between getting odds to beat one player who can often be beaten with one pair, and getting odds that looks the same to try to come up with the best hand out of several. Offsuit disconnected hands do worse in multiway pots relative to par than they do in heads-up pots. That there are several players limping in should make you fold trash hands even faster than when you aren't getting odds that look as good, particularly if the limpers are somewhat selective.
"Position, position, position!"
"It's a kicker game."
These mantras argue against playing trash from the small blind.