Simple response format because the quote gore is getting too much @Lafleursguy
1. The roster as it is will not have a substantially better performance than last year (the Weak Roster argument)
That's fine but not the optimal course of action (ie ReHabs is not being impatient; we're just having hockey talk in July)
2. It would be better to improve the roster and enjoy more success and more meaningful games in the regular season (the Incremental Progress argument)
3. Success in the playoffs is generally predicated on experience in the playoffs; experience in the playoffs is predicated on success in the regular season (the No-Flip-Switch argument)
4. To be successful in the playoffs this budding team needs to experience regular season success (the Taste-of-the-Playoffs-is-Valuable argument)
5. Since the Habs have finished bottom5 for two years straight, by definition the top6, bottom6, and d-corps could all be improved (the Rebuilding-requires-Building argument)
6. The top6 is bad because: a) Dach is extremely injury prone and unproven as a secondary scorer, b) Roy is unproven and a long-shot bet, and c) there is no 7th option top6 player in the Habs depth chart in case of injury or underperformance.* (the Bad Top6 argument)
7. The availability of Patrick Laine at a relatively low cost is itself a worthwhile gambit to improve the top6 and is worth pursuing because it would give the Habs sorely needed scoring depth
8. Patrick Laine is not the only way to improve the top6: there is also Rutger McGroarty who is in a completely different situation and has a completely different player profile. There is also Necas, and Zegras, and who knows who else could be available. Some would say there is also Max Pacioretty but at this point there are other considerations and diminishing returns.
Improving the roster with a two-year commitment is almost perfectly in line with what Hughes outright said he wants to do. I believe that's what Hughes sought to do with Marchessault who would've been a more safe gamble than Patrick Laine.
Because there are interesting prospects coming into the picture, the Habs do not need to make long-term or "transformative" commitments until they know how these prospects will fare in the NHL and their trajectory. So as a bridge to help continue the momentum and the gains in pt% and performance, a short-term cap commitment to a top6 player added to the roster will help to 1) improve the roster/top6, 2) contribute to win more points and ensure the Habs are in the playoff race for longer and therefore play more meaningful games, and to a lesser extent 3) raise the trade value of the players around him.
As an aside, I already made the argument that the 6th best forward of a winning team is often considerably more productive than a 32pt forward. The comment seems to have been deleted so I won't make the argument or present the stats again -- I'm sure you can understand.
*I don't even know who would be the first call-up for a skill forward role... 5'9" 174lbs Sean Farrell and his 0.5 AHL PPG?