I am doing a reasearch on light verbs and wondered if anyone could give some opinions on why people tend to use light verb cosntructions, such as "take a shower", "make a decision" etc., over the simple/full verbs, for example "shower" or "decide"?
The following is what I see.
As pointed out by @jlawler above, such constructions are often considered alongside other periphrastic constructions. It is believed that the main purpose they serve is making speech more expressive. Hence, you need to consider the reasons why this necessity typically arises. I guess it has something to do with phonetic processes like reduction.
It should be pointed out that LVC are very common to language learners. If the inflectional system of a language in question appears quite complex, at the first it is far more natural to learn a small number of light verbs by heart together with deverbal nouns (which, as all other nouns, are easier to memorize) to start talking. I admit that in some cases the LVCs can be calqued from the corresponding constructions in the first language.
The usage may also be explained by fashion or tradition. It happens when LVC are attributable to a certain functional style or a dialect (so-called prestige form) which is to be used given the circumstances.
The cause may be certain grammatical specifics of a given language. For example, in Russian there is no 1-Sg-Fut form of the verb pobedit' - 'win', so instead of saying ja pobediu or ja pobezhdu - 'I will win' (see below)*, one would say ja oderzhu pobedu - 'I will take victory'.
*The reason for the absence is ceasing of a phonological process, which once obscured the alternation /di/~/zhd/ (metathesis from /dzh/) whereby pobezhdu lost its connection to pobedit'. Furthemore, forms with oderzhat' - 'take' are spreading to other persons and tenses.
Per your request, this is only an opinion...
Some speakers will align a question's construction pattern with an answer. The LVC question, "Did you make a decision?" Will result in the LVC response, "Yes, I made a decision."
Single word imperatives like "decide" are often considered time-sensitive orders. From a pragmatics of politeness perspective, "make a decision" is considered more polite than "decide". This is the brevity vs. loquacious argument, where indirectness is used to absorb burdens associated with questions or requests.
Note that other LVC's don't emphasize politeness, but they do attempt to hedge/diminish the concept: "I had a smoke" (diminishes the action related to smoking) "We took a loss" (diminished the action of lose)
In some cases, the nominative form is much more common, and is the cognitively easy way to describe something: "I took a cab" vs. "I cabbed"
Some appear to be LVC's but either lack a reasonable paraphrase, or they put emphasis on the direct object. Hence, they're LVC out of necessity. "He took the gold"
Some carry extra semantic content: "They made an entrance" vs. "they entered"; "She made a dash" vs. "She dashed"
Nominative form enables better counting: "He made a joke" (count=1) vs. "he joked" (verb=past;aspect=singular, count=?)
Again - just my opinions.